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FEB 2 8 1988

3QC FASTijR H OP

YALE UNIVERSITY
MRS. HEPSA ELY SILLIMAN MEMORIAL LECTURES

THE INTEGRATIVE ACTION OF THE
NERVOUS SYSTEM

SnxDiAN Memorial Lectures

PDBLISHSD BT TALB UUXVERSITY PRESS

Electricity and Matter. By Pro-
fessor J. J. Thomson. Net, I1.25.

The Integrative Action of the Nerv-
ous System. By Professor C. S.
Sherrington. Net, $3.50.

Radioactive Transformations. By
Professor E. Rutherford. Net, 13.50.

Experimental and Theoretical
Applications of Thermodynamics
TO Chemistry. By Professor Walther
Nemst. Net,%\.2^,

THE

INTEGRATIVE ACTION

OF THE

NERVOUS SYSTEM

CHARLES S. SHERRINGTON

D.Sc, M.D., HoNrijLD. ToE^ F.R.S.

Htit Frtfissar tf PkysMogy in the UtdversUy of Ltverpoei,

Umtarmry Marnier of the Awurkan Pkyshkgkai SodOy,

6v,

WITH ILLUSTRATIONS

mw haven: tale uniyebsitt pbbss

LONDON: HBNBT FBOWDE

OXFORD UNIYEBSITT FBE88

MCMXI

PuUished (ktoher. iqot

To
DAVID FERRIER

IN TOKEN OF RECOGNinON OF BIS MANY SERVICES

THE EXPERIMENTAL PHYSIOLOGY

THE CENTRAL NERVOUS SYSTEM

42121

THE SILLIMAN FOUNDATION

In -the year 1883 a legacy of eighty thousand dollars
was left to the President and Fellows of Yale College in
the city of New Haven, to be held in trust, as a gift
from her children, in memory of their beloved and hon-
ored mother Mrs. Hepsa Ely Silliman.

On this foundation Yale College was requested and
directed to establish an annual course of lectures de-
signed to illustrate the presence and providence, the
wisdom and goodness of God, as manifested in the natural
and moral world. These were to be designated as the
Mrs. Hepsa Ely Silliman Memorial Lectures. It was
the belief of the testator that any orderly presentation
of the facts of nature or history contributed to the end
of this foundation more eflFectively than any attempt to
emphasize the elements of doctrine or of creed ; and he
therefore provided that lectures on dogmatic or polemical
theology should be excluded from the scope of this foun-
dation, and that the subjects should be selected rather
from the domains of natural science and history, giving
special prominence to astronomy, chemistry, geology, and
anatomy.

It was further directed that each annual course should
be made the basis of a volume to form part of a series
constituting a memorial to Mrs. Silliman. The memo-
rial fund came into the possession of the Corporation
of Yale University in the year 1902; and the present
volume constitutes the second of the series of memorial
lectures. The first volume in this series was " Electricity
and Matter," by Prof. J. J. Thomson, of Cambridge
University.

PREFACE

The pressure of varied work has prevented my forward-
ing the text of these lectures for publication so early as
I could have wished, and I take this occasion of express-
ing my regret at the delay. The circumstance that
thus impels me to preface with a few words their issue
affords me also the opportunity of recording how much
I am indebted to President Hadley and the authorities
of Yale University for their kindness during a visit
which I shall ever remember with pleasure. To Pro-
fessor Chittenden, Director of the Sheffield Scientific
School of the University, I owe further a debt of grati-
tude for unstinted assistance open to me from him on
all occasions.

C. S. S.

Pagb

CONTENTS

LECTURE I

INTRODUCTORY— CO-ORDINATION IN THE SIMPLE

REFLEX I

Argument: The Jiervous system and the integration of bodily
reactions. Characteristics of integration by nervous agency.
The unit mechanism in integration by the nervous system is
the reflex. Co-ordination of reflexes one with another. Co-
ordination in the simple r^ex. Conduction in the reflex-arc.
Function of the receptor to lower for its reflex-arc the threshold
value of one kind of stimulus and to heighten the threshold
value of all other kinds of stimuli for that arc : it thus confers
selective excitability on the arc. Diflerences between con-
duction in nerve-trunks and in reflex-arcs respectively. These
probably largely referable to the intercalation of synaptic mem-
branes in the conductive mechanism of the arc. Latent time
of reflexes. Reflex latency inversely proportional to intensity
of stimulation. Latency of initial and incremental reflexes.
None of the latent interval consumed in establishing connec-
tion between the elements of a resting arc After-discharge a
characteristic of reflex reactions. Increase of after-discharge
by intensification of the stimulus, or by prolongation of short
stimulL " Inertia " and " momentum " of reflex-arc reactions.

LECTURE II

CO-ORDINATION IN THE SIMPLE REFLEX {continued) 36
Argument: Reflex-arcs show high capacity for summing excita-
tions. Irreversibility of direction of conduction in reflex-arcs.
Reversibility of direction of conduction in certain nerve-nets,
e. g. that of Medusa. Independence between the rhythm of
the reflex-discharge and the rhythm of the external stimulus
exciting it. Refractory phase in reflexes ; in the eyelid-reflex ;
in the scratch-reflex. The neuronic construction of the reflex-
arc of the scratch-reflex. Long descending proprio-spinal
tracts revealed by the method of "successive degeneration."
The "final common path" and the "afferent arc." Intra-
spinal seat of the refractory phase of the scratch-reflex.
The value of refractory phase in the co-ordination of the
swimming of Medusa. Its value in the co-ordination of
the scratch-reflex. Significance of the intraspinal situation of
the refi-actory phase of the scratch-reflex. Other instances
of "central" refractory phase.

xii CONTENTS

LECTURE III

Page
CO-ORDINATION IN THE SIMPLE REFLEX {concluded) 70

Argument: Correspondence between intensity of stimulus and
intensity of reflex reaction. Differences between different re-
flexes in this respect Functional solidarity of the intraspinal
group of elements composing a reflex " centre." Sensitivity
of reflexes, as compared with nerve-trunks, to asphyxial and
anaemic conditions, and to anaesthetic and certain other drugs.
Functional significance of the neural perikarya. Reflexes
of double-sign. Reflexes of successive double-sign, and of
simultaneous double-sign. Evidence of reciprocal innervation
in reflexes. Reflex inhibition of the tonus of skeletal muscles.
Reflex inhibition of the knee-jerk. Time-relations and other
characters of reflex inhibition as exemplified by the flexion-
reflex. Other examples of inhibition as part of reflex recipro-
cal innervation. The seat of this reflex inhibition is intraspinal.
Conversion of reflex inhibition into reflex excitation by strych-
nine and by tetanus toxin. Significance of the " central "
situation of reflex inhibition in the cases here dealt with.

LECTURE IV

INTERACTION BETWEEN REFLEXES 114

Argument: The "simple reflex" a convenient but artificial ab-
straction. Compounding of reflexes. The principle of the
common path. Relative aperiodicity of the final common path.
Afferent arcs which use the same final common path to dif-
ferent effect have successive but not simultaneous use of it
" Allied " reflexes. Allied reflexes act harmoniously, are
capable of simultaneous combination, and in many cases
reinforce one another's action on the final common path.
"Antagonistic" reflexes.* Alliance or coalition occurs be-
tween (i) individual reflexes belonging to the same "type-
reflex," (2) certain reflexes originated by receptors of different
species but situate in the same region of surface, (3) certain
reflexes belonging to proprioceptive organs secondarily ex-
cited by reflexes initiated at the body-surface (the three
fields of reception, extero-c^ptive, intero-ceptive, and proprio-
ceptive), (4) certain reflexes initiated from widely separate but
functionally interconnected body-regions. Alliance between
reflexes exemplified in inhibitory actions as well as in excitatory.
Antagonistic reflexes interfere, one reflex deferring, interrupt-
ing, or cutting short another, or precluding the latter altogether
from taking effect on the final common path. Intraspinal

CONTENTS xiii

Page
seat of the interference. Compound reflexes may interfere in
part. The place (? synapse) where convergent afferent paths
impinge on a common path constitutes a mechanism of co-
ordination. The convergence of afferent paths to form com-
mon paths occurs with great frequency in die central nervous
sjTStem. A quesUon whether any reflexes are in the intact
organism wholly neutral one to another.

LECTURE V

COMPOUND REFLEXES: SIMULTANEOUS COMBINA-

TION 150

Argument: Combination of reflexes simultaneously proceeding,
^iread of reflex-response about a focus. Gray matter and
fines of reflex resistance. " Short " reflexes and " long " re-
flexes. Rules decipherable in the spread of reflex reaction.
Pfluger's " laws " of spinal irradiation. The " reflex figure."
VariabiUty of reflex result Irradiation of a reflex attaches
itself to the problem of the simultaneous combination of
reflexes. Co-ordination of reflex result obtains even when
large mixed afferent nerve-trunks are stimulated. The move-
ment excited by stimulation of the motor spinal nerve-root
does not really resemble a movement evoked reflexly or by
the win. Extent of simultaneous combinations of reflexes.
Simultaneous stimuU arrange themselves naturally in constel-
lations in which some component is usually of pre-eminent
intensity. The resulting compound reaction has both positive
and negative sides.

LECTURE VI

COMPOUND REFLEXES: SUCCESSIVE COMBINATION 181

Argument: Co-ordination of reflex sequences. Chain-reflexes
(Loeb). Overlapping of successive* stimuli in time. The
sequence of allied reflexes. Spread of bahnung, '* immediate
induction." Sequence of antagonistic reflexes. The r6le of
inhibition in this transition. Views of the nature of inhibition :
Rosenthal, Wundt, E. Hering, Gaskell, Verwom, J. S. Mac-
donaki The "interference" of reflexes. "Alternating re-
flexes." W. Macdougall's view of "drainage of energy."
" Compensatory reflexes." Factors determining the issue of
the competition between antagonistic reflexes. " Successive
induction." Rebound-effects in spinal reactions; tend to
restore reflex equitibrium. Fatigue in reflexes. Relative high
resistance to fiitigue possessed by the final common path,
L e. motor neurone. Intensity of reaction a decisive factor in

xiv ^^^^ CONTENTS

Page

the competition of afferent arcs for possession of the final
common path. Noci-ceplive nerves. Prepotency of reflexes
generated by receptors that considered as sense organs initiate
sensations with strong aflfeclive tone. Resistance of tonic
reflexes to fatigue. Ail these factors render the conductive
pattern of the central nervous system mutable between certain
limits.

LECTURE VII

REFLEXES AS ADAPTED REACTIONS TsT

Argument: Reflexes as adapted reactions. The purposes of
various type-reflexes. Shock a difficulty in deciphering the
purpose of reflexes* Characters of spinal shock. Its inci-
dence conflned to the aboral side of the transection. Its
difference in severity in different reflexes and in different
animals* Shock referable not to the irritation of the trauma
but to the cutting off by the trauma of some supra-spinal
influence.

Pseudaffective reflexes afford opportunity for determining
the pain-path in the spinal cord. This ascends both lateral
columns, chiefly the one crossed from side of stimulation.
The "chloroform cry " in decerebrate animals. Mimesis of
pleasure as compared with mimesis of pain. The bodily res-
onance of the emotions. The theory of James, Lange, and
Sergi* Emotional expressions in dogs deprived of visceral
and largely of bodily sensation.

LECTURE VIII

^,^— €OME ASPECTS OF THE REACTIONS OF THE

MOTOR CORTEX • . . . , 26

Argument: Remarkable that electrical stimuli applied to the
organ of mentality yield Tfinth regularity certain localized
movements from certain restricted areas of its surface. Func-
tional topography of *' motor " cortex in the chimpanzee,
orang-utan, and gorilla. The cerebral fissures, not functional
boundaries. The anthropoid ape has a direct pyramidal tract
like that of man. Recovery of function not due to symmetrical
part of opposite hemisphere taking on supplemental work*
Inhibition as elicitable from the cortex. Reciprocal innervation
of antagonistic eye-muscles. Reciprocal inhibition in other
muscular groups. Seat of the inhibition subcortical in these
cases. Reciprocal innervation in willed movements. Pre-
ponderant representation in the "motor" cortex of the same
movements as are preponderantly eUcitable as local reflexes

CONTENTS XV

Page
from the cord and bulb. Scanty representation of certain
movements as cortical and local spinal reactions alike. Ap-
pearance under strychnine and tetanus toxin of movements
reversing the normal direction of the preponderance. This
due to these agents transmuting reciprocal inhibition into
excitation. Dectrebraie rigidity. A system of tonic innerv'a-
tion in action. Strychnine and tetanus toxin augment this
innervation. Hughlings Jackson's " co-operative antagonism "
of paired systems of innervation, one tonic, the other phasic.
Decerebrate rigidity and hemiplegic rigidity. The relation of
the cortex to receptor organs ; the pre-eminent representation
in it of the " distance-receptors."

LECTURE IX

'THE PHYSIOLOGICAL POSITION AND DOMINANCE

OF THE BRAIN 308

^rgitmaii: The primitive reflex arc. The difiiise nervous sys-
tem and the gray-centred nervous system ; the central nervous
system a part of the latter. Nervous integration of the seg-
ment The three receptive fields. Richness of the extero-
ceptive field. Special refinements of the receptor-organs of
the "leading" segments. The refined receptors of the
leading segments are "distance-receptors." "Distance-
receptors " ; the projidence of sensations. Extensive inter-
nuncial paths belonging to "distance-receptors." " Distance-
receptors " initiate precurrent reactions. Consummatory
reactions ; strong affective tone of the sensations adjunct to
them. Receptive range and locomotion. The " head " as
physiologically conceived. Proprio-ceptive arcs excited sec-
ondarily to other arcs. Qose functional connection between
the centripetal impulses fix>m muscles and from the labyrinth.
Tonic reflexes (of posture, etc.) and compensatory reflexes
are characteristic reactions of this combined system. Nervous
integration of the segmental series. Restriction of segmental
distribution a £u:tor in bodily integration. The cerebellum
is the main ganglion of the proprio-ceptive system. The
cerebrum is the ganglion of the " distance-receptors."

LECTURE X

SENSUAL FUSION 354

Arpament: Nervous integration in relation to bodily movement
and to sensation compared. Sensual fusion in a relatively
simple instance of binocular vision. The rotating binocular
lantern. Flicker sensations generated at "corresponding retinal

XVI CONTENTS

Page

points " ; absence of evidence of their summation or interfer-
ence either with synchronous or asynchronous flicker of similar
frequency. Their interference when the flicker is of dissimilar
frequency. Talbot's law not applicable to "corresponding
points." Fechner's paradox. IVevalence of contours under
Weber's law and under binocular summation compared. The
physiological initial stages of the reaction generated in either
of a pair of corresponding retinal points proceeds without
touching the apparatus of the twin point Only afler the sen-
sations initiated from the right and left " points " have been
elaborated so far as to be well amenable to introspection does
interference between the reactions of the two (right and left)
eye-systems occur. The convergence of nerve-paths from the
right and left retinae respectively toward one cerebral region
is significant of union for co-ordination of motor reaction
rather than for synthesis of sensation. Resemblances between
motor and sensual reactions. The cerebmm pre-eminently
the organ of and for the adaptation of reactions.

BIBLIOGRAPHICAL REFERENCES 395

INDEX 403

THE INTEGRATIVE ACTION OF
THE NERVOUS SYSTEM

LECTURE I

INTRODUCTORY — CO-ORDINATION OF THE SIMPLE

REFLEX

Argument: The nervous system and the integration of bodily reactions.
Characteristics of integration by nervous agency. The unit roechan-
ism in integration by the nervous Sjrstem is the reflex. Co-ordination
of reflexes one with another. Co-ordination in the simple r^ex.
Conduction in the reflex-arc. Function of the receptor to lower for
its reflex-arc the threshold value of one kind of stimulus and to
heighten the threshold value of all other kinds of stimuli for that
arc: it thus confers selective excitability on the arc. Differences
between conduction in nerve-trunks and in reflex-arcs respectively.
These probably largely referable to the intercalation of synaptic mem-
branes in the conductive mechanism of the arc Latent time of
reflexes. Reflex latency inversely proportional to intensity of stimu-
lation. Latency of initial and incremental reflexes. None of the
latent interval consumed in establishing connection between the
elements of a resting arc After-discharge a characteristic of reflex re-
actions. Increase of after-discharge by intensification of the stimulus,
or by prolongation of short stimuli. " Inertia " and " momentum "
of reflex-arc reactions.

Nowhere in physiology does the cell-theory reveal its presence
more frequently in the very framework of the argument than at
the present time in the study of nervous reactions. The cell-
theory at its inception depended for exemplification largely on
merely morphological observations ; just as these formed origi-
nally the almost exclusive texts for the Darwinian doctrine of
evolution. But with the progress of natural knowledge, biology
has passed beyond the confines of the study of merely visible
form, and is turning more and more to the subtler and deeper
sciences that are branches of energetics. The cell-theory and
the doctrine of evolution find their scope more and more, there-
fore, in the problems of function, and have become more and

2 INTRODUCTORY [Lect.

more identified with the aims and incorporated among the
methods of physiology.

The physiology of nervous reactions can be studied from
three main points of view.

In the first place, nerve-cells, like all other cells, lead indi-
vidual lives, — they breathe, they assimilate, they dispense their
own stores of energy, they repair their own substantial waste ;
each is, in short, a living unit, with its nutrition more or less
centred in itself Here, then, problems of nutrition, regarding
each nerve-cell and regarding the nervous system as a whole,
arise comparable with those presented by all other living cells.
Although no doubt partly special to this specially differentiated
form of cell-life, these problems are in general accessible to the
same methods as apply to the study of nutrition in other cells and
tissues and in the body as a whole. We owe recently to Verwom
and his co-workers advances specially valuable in this field.

Secondly, nervous cells present a feature so characteristically
developed in them as to be specially theirs. They have in ex-
ceptional measure the power to spatially transmit (conduct)
states of excitement (nerve-impulses) generated within them.
Since this seems the eminent functional feature of nerve-cells
wherever they exist, its intimate nature is a problem co-extensive
with the existence of nerve-cells, and enters into every question
regarding the specific reactions of the nervous system. This
field of study may be termed that of nervi-cell conduction.

But a third aspect which nervous reactions offer to the physi-
ologist is the integrative. In the multicellular animal, especially
for those higher reactions which constitute its behaviour as a
social unit in the natural economy, it is nervous reaction which
par excellence integrates it, welds it together from its compo-
nents, and constitutes it from a mere collection of organs an
animal individual. This integrative action in virtue of which the
nervous system unifies from separate organs an animal possess-
ing solidarity, an individual, is the problem before us in these
lectures. Though much in need of data derived from the two
previously mentioned lines of study, it must in the meantime be
carried forward of itself and for its own sake.

rj INTEGRATION BY NERVOUS AGENCY 3

The integration of the animal organism is obviously not the
result solely of any single agency at work within it, but of several
Thus, there is the mechanical combination of the unit cells of the
individual into a single mass. This is effected by fibrous stro-
mata, capsules of organs, connective tissue in general, e. g. of
the liver, and indeed the fibrous layer of the skin encapsulating
the whole body. In muscles this mechanical integration of the
organ may arrive at providing a single cord tendon by which
the tensile stress of a myriad contractile cells can be additively
concentrated upon a single place of application.

Integration also results from chemical agency. Thus, repro-
ductive organs, remote one from another, are given solidarity
as a system by communication that is of chemical quality;
lactation supervenes post pattum in all the mammary glands of
a bitch subsequent to thoracic transection of the spinal cord
severing all nervous communication between the pectoral and
the inguinal mammae (Goltz>. In digestive organs we find
chemical agency co-ordinating the action of separate glands,
and thus contributing to the solidarity of function of the diges-
tive glands as a whole. The products of salivary digestion on
reaching the pyloric region of the stomach, and the gastric
secretion on reaching the mucosa of the duodenum, make there
substances which absorbed duly excite heightened secretion of
gastric and of pancreatic juice respectively suited to continue
tfie digestion of the substances initiating the reaction (Bayliss and
Starling, Edkins). Again, there is the integrating action effected
by tlie circulation of the blood. The gaseous exchanges at one
limited surface of the body are made serviceable for the life
of every living unit in the body. By the blood the excess ol
heat produced in one set of organs is brought to redress the
loss of heat in others; and so on.

But the integrative action of the nervous system is different
from these, in that its agent is not mere intercellular material,
as in connective tissue, nor the transference of material in mass,
as by the circulation ; it works through living lines of stationary
cells along which it despatches waves of physico-chemical dis-
turbance, and these act as releasing forces in distant organs

4 INTRODUCTORY [Lect.

where they finally impinge. Hence it is not surprising that
nervous integration has the feature of relatively high speedy a
feature peculiarly distinctive of integrative correlation in animals
as contrasted with that of plants, the latter having no nervous
system in the ordinary sense of the word.

The nervous system is in a certain sense the highest expres-
sion of that which French physiologists term the milieu interne.
With the transition from the unicellular organism to the multi-
cellular a new element enters general physiology. The phe-
nomena of general physiology in the unicellular organism can
be divided into two great groups ; namely, those occurring within
the cell, intracellular, and those occurring at the surface of the
cell, in which forces that are associated with surfaces of separa-
tion have opportunity for play at the boundary between the
organism and its environment But in the multicellular organ-
ism a third great group of phenomena exists in addition ; namely,
those which are m/^fcellular, occurring in that complex material
which the organism deposits in quantity in the intercellular inter-
stices of its mass as a connecting medium between its individual
living units.

When the intercellular substance is solid, e. g. in many con-
nective tissues, the physiological agencies for which it affords a
field of operation are mechanical rather than chemical. The
organism obtains from it scaffolding for supporting its weight,
levers for application of its forces, etc., and in this degree the
intercellular material performs an integrative function. Where
the intercellular material is fluid, as in blood, lymph, and tissue
juice, it constitutes a field of operation for agencies chemical
rather than mechanical. The intricacy of the chemistry of this
milieu interne is shown by nothing better than by the specificity
of the precipitins, etc., the intercellular media for each separate
animal species yielding its own particular kinds. The cells of a
multicellular organism have therefore in addition to an environ-
mental medium in which the organism as a whole is bathed, and
to which they react either directly or through the medium of
surface cells, an internal medium created by their organism itself
and in many respects specific to itself.

II RECEPTION, CONDUCTION, AND END-EFFECT 5

But the internal interconnection of the multicellular organism
is not restricted to intercellular material. Intercellular material
is, after all, no living channel of communication, delicately re-
sponsive to living changes though it may be. An actually living
internal bond is developed. When the animal body reaches some
degree of multicellular complexity, special cells assume the ex-
press office of connecting together other cells. Such cells, since
their function is to stretch from one cell to another, are usually
elongated ; they form protoplasmic threads and they intercon-
nect by conducting nervous impulses. And we find this living
bond the one employed where, as said above, speed and nicety
of time adjustment are required, as in animal movements, and
also where nicety of spatial adjustment is essential, as also in
animal movements. It is in view of this interconnecting func-
tion of the nervous system that that field of study of nervous
reactions which was called at the outset the third or integrative,
assumes its due importance. The due activity of the intercon-
nection resolves itself into the co-ordination of the parts of the
animal mechanism by reflex action.

It is necessary to be clear as to what we understand by the
expression ** reflex " action.

In plants and animals occur a number of actions the initia-
tion of which is traceable to events in their environment The
event in the environment is some change which acts on the organ-
ism as an exciting stimulus. The energy which is imparted to
the organism by the stimulus is often far less in quantity than
the energy which the organism itself sets free in the movement
or other effect which it exhibits in consequence of the applica-
tion of the stimulus. This excess of energy must be referred to
enei^ potential in the organism itself. The change in the
environment evidently acts as a releasing force upon the living
machinery of the organism. The source of energy set free is
traced to chemical compounds in the organism. These are of
high potential value, and in immediate or mediate consequence
of the stimulus decompose partly, and so liberate external from
internal energy. It is perfectly conceivable, and in many undif-
ferentiated organisms, especially in unicellular, e, g. amoeba, is

6 INTRODUCTORY [Lect.

actually the case, that one and the same living structure not
only undergoes this physico-chemical change at the point at
which an external agent is applied, but is subject to spread of
that change from particle to particle along it, so that there then
ensue in it changes of form, movement In such a case the
initial reaction or reception of the stimulus, the spatial transmis-
sion or conduction of the reaction, and the motor or other end'
effect^ are all processes that occur in one and the same living
structure. But in many organisms these separable parts of the
reaction are exhibited by separate and specific structures. Sup-
pose an animal turn its head in response to a sudden light
Large fields of its body take part in the reaction, but also large
fields of it do not Some of its musculature contracts, particu-
larly certain pieces of its skeletal musculature. The external
stimulus is, so to say, led to them by certain nerves in the
altered form of a nervous impulse. If the neck nerves are
severed the end-effect is cut out of part of the field ; and the
nerves themselves cannot exhibit movement on application of
the stimulus. The optic nerve itself is unable to enter into a
heightened phase of its own specific activity on the application
of light Initiation of nervous activity by light is the exclusive
(in this instance) function of cells in the retina, retinal receptors.
In such cases there exist three separable structures for the three
processes — initiation^ conduction, and end-effect.

These reactions, in which there follows on an initiating reac-
tion an end-effect reached through the mediation of a conductor,
itself incapable either of the end-effect or, under natural condi-
tions, of the inception of the reaction, are " reflexes." The con-
ductors arc nerve. Usually the spaces and times bridged across
by the conductors arc quite large, and easily capable of measure-
ment Now there occur cases, especially within the unicellular
organism and the unicellular organ, where the spaces and times
bridged are minute. In them spread of response may involve
"conduction" (Poteriodendron, Vorticella) in some degree
specific. Yet to cases where neither histologically nor physi-
ologically a specific conductor can be detected, it seems better
not to apply the term " reflex." It seems better to reserve that

1] CO-ORDINATION OF REFLEXES 7

expression for reactions employing specifically recognizable
nerve-processes and morphologically differentiated nerve-cells;
the more so because the process of conduction in nerve is
probably a specialized one, in which the qualities of speed and
fineedom fix>m inertia of reaction have been attained to a degree
not reached elsewhere since not elsewhere demanded.

The conception of a reflex therefore embraces that of at
least three separable structures, — an effector organ, e.g.^ gland
cells or muscle cells; a conducting nervous path or conductor
leading to that organ ; and an initiating organ or receptor whence
the reaction starts. The conductor consists, in the reactions
which we have to study, of at least two nerve^rells, — one con-
nected with the receptor, the other with the effector. For our
purpose the receptor is best included as a part of the nervous
system, and so it is convenient to speak of the whole chain of
structures — receptor, conductor, and effector — as a reflex-arc.
All that part of the chain which leads up to but does not include
the effector and the nerve-cell attached to this latter, is conven-
iently distinguished as the afferetU-^rc.

The reflex-arc is the unit mechanism of the nervous system
when that system is regarded in its integrative function. The
umt reaction in nervous integration is the reflex, because every
reflex is an integrative reaction and no nervous action short of a
reflex is a complete act of integration. The nervous synthesis
of an individual from what without it were a mere aggregation
of commensal organs resolves itself into co-ordination by reflex
action. But though the unit reaction in the integration is a
reflex, not every reflex is a unit reaction, since some reflexes
are compounded of simpler reflexes. Co-ordination, therefore,
is in part the compounding of reflexes. In this co-ordination
there are therefore obviously two grades.

ThA tffmple reflex. There is the co-ordination which a reflex
action introduces when it makes an effector organ responsive to
excitement of a receptor, all other parts of the organism being
supposed indifferent to and indifferent for that reaction. In this
grade of co-ordination the reflex is taken apart, as if separable
firom all other reflex actions. This is the simple reflex, A

8 THE SIMPLE REFLEX [Lect.

simple reflex is probably a purely abstract conception, because
all parts of the nervous system are connected together and no
part of it is probably ever capable of reaction without affecting
and being affected by various other parts, and it is a system
certainly never absolutely at rest. But the simple reflex is a
convenient, if not a probable, fiction. Reflexes are of various
degrees of complexity, and it is helpful in analyzing complex
reflexes to separate from them reflex components which we
may consider apart and therefore treat as though they were
simple reflexes.

In the simple reflex there is exhibited the first grade of co-
ordination. But it is obvious that if the integration of the
animal mechanism is due to co-ordination by reflex action,
reflex actions must themselves be co-ordinated one with another ;
for co-ordination by reflex action there must be co-ordination of
reflex actions. This latter is the second grade of co-ordination.
The outcome of the normal reflex action of the organism is an
orderly coadjustment and sequence of reactions. This is very
patently expressed by the skeletal musculature. The co-ordina-
tion involves orderly coadjustment of a number of simple reflexes
occurring simultaneously^ i, e. a reflex pattern, figure, or " com-
plication," if one may warp a psychological term for this use ;
orderly succession involves due supercession of one reflex by
another, or of one group of reflexes by another group, /. e.
orderly change from one reflex pattern or figure to another.
For this succession to occur in an orderly manner no com-
ponent of the previous reflex may remain which would be out
of harmony with the new reflex that sets in. When the change
from one reflex to another occurs it is therefore usually a far-
reaching change spread over a wide range of nervous arcs.

This compounding of reflexes with orderliness of coadjust-
ment and of sequence constitutes co-ordination, and want of it
inco-ordination. We may therefore in regard to co-ordination
distinguish co-ordination of reflexes simultaneously proceeding,
and co-ordination of reflexes successively proceeding. The
main secret of nervous co-ordination lies evidently in the
compounding of reflexes.

I] THE RECEPTOR 9

Co-ordlnatioii in the simple reflex. It is best to clear the
way toward the more complex problems of co-ordination by
considering as an earlier step that which was termed above,
the first grade of co-ordination, or that of the simple reflex.
From the point of view of its office as integrator of the
animal mechanism, the whole function of the nervous system
can be summed up in the one word, conduction. In the simple
reflex the evidence of co-ordination is that the outcome of
the reflex as expressed by the activity induced in the eflector
organ is a response appropriate to the stimulus imparted to the
receptor. This due propriety of end-effect is largely traceable
to the action of the conductor mediating between receptor and
afiector. Knowledge of the features of this " conduction " is
therefore a prime object of study in this connection.

But we have first to remember that in dealing with reflexes
even experimentally we very usually deal with them as reactions
for which the reflex-arc as a whole and without any separation
into constituent parts is laid under contribution. The reflex-
arc thus taken includes the receptor. It is assuredly as truly a
functional part of the arc as any other. But, for analysis of the
arc's conduction, it is obvious that by including the receptor we
are including a structure which, as its name implies, adaptation
has specialized for excitation of a kind different from that
obtaining for all the rest of the arc. It is therefore advanta-
geous, as we have to include the receptor in the reflex-arc, to
consider what characters its inclusion probably grafts upon the
functioning of the arc.

Marshall Hall*^* drew attention to the greater ease with
which reflexes can be elicited from receptive surfaces than from
afferent nerve-trunks themselves ; and this has often been con-
firmed (Eckhard, Biedermann). Steinach ^^ has measured the
lowering of the threshold value of stimulation when in the frog
a reflex is elicited by a mechanical stimulus applied to skin
instead of to cutaneous afferent nerve. The lowering is con-
siderable. There are numerous instances in which particular
reflexes can be elicited from the receptive surface by particular

* The reference nambers in the text refer to the bibliographical list at the
cod of the volume.

lo THE SIMPLE REFLEX [Lect.

stimuli only. Goltz** endeavoured in vain to evoke the reflex
croak of the female frog by applying to the skin electrical stim-
uli. Mechanical stimuli of non-nocuous kind were the only stim-
uli that proved effective. From the afferent nerve itself by direct
stimulation the reflex could but rarely be elicited at all. Later
Goltz's pupil Gergens ^ succeeded in provoking the reflex by ap-
plying to the skin a mild discharge from an influence machine.

A remarkable reflex ^^ is obtainable from the planta of the
hind foot in the " spinal " dog. The movement provoked is a
brief strong extension at knee, hip, and ankle. This is the
" extensor-thrust" It seems obtainable only by a parttctilar
kind of mechanical stimulation. I have never succeeded in
eliciting it by any form of electrical stimulation, nor by any
stimulation applied directly to an afferent nerve-trunk.

Again, a very characteristic reflex in the cat is the pinna-
reflex.*^ If the tip of the pinna be squeezed, or tickled* or in
some cases even touched, the pinna itself is crumpled so that
its free end is turned backward, as in Darwin's ^ picture of a
cat prepared to attack. The afferent nerve of this reflex appears
to be in part at least not the cranial flfth nerve, but the foremost
cervical. The reflex emerges very early from the shock of de-
cerebration and is submerged very late in chloroform narcosis.
This reflex, easily elicitable as it is by various mechanical stimuli
to the skin, I have never succeeded in provoking by any form of
electrical stimulation.

The same sort of difference, though less marked in degree^ is
exhibited by the scratch-reflex.®^. i», ^\ aw, wo Thjs reflex is
one in which various forms of innocuous mechanical stimula-
tion (rubbing, tickling, tapping) applied to the skin of the
back behind the shoulder evoke a rhythmic flexion (scratching
movement) of the hind limb, the foot being brought toward
the seat of stimulation. This reflex in the spinal dog, although
usually elicitable, varies much under various circumstances in its
degree of elicitability. When easily elicitable it can be evoked
by various forms of electrical stimulation as well as by mechan-
ical ; but when not easily elicitable electrical stimuli altogether
fail, while rubbing and other suitable mechanical stimuli still
evoke it, though not so readily or vigorously as usual.

^^^^^^ ADEQUATE SlIMULUS ^^^^^^B

A c\^estion germane to this is the oft-dehated sensitivity of H
laiious internal organs. Direcrt stimulation of various afferent H
ttCTves of the visceral system is itself well known to yield H
Tcfiexes on blood-pressure, etc. But in regard to the sensitivity H

1 ^^i *^ , ^i^,,,,^

tar ^^H

OB ^^^H

4 *^vW ^, 1

i^^

^^^r

■ pr^z^ced, in the cn under CHCt and ctuare, a|i
■>ject»OQ of a,5 cc of saline salution in the asmm
m» duct, 1 Ittie marked below in seconds id, BibUoi

12 THE SIMPLE REFLEX [Lect.

of the organs themselves we have, on the one hand, the passage
of bilestones, renal calculi, etc., accompanied by intense sensa-
tions, and on the other hand the insensitivity of these ducts and
various allied visceral parts as noted by Haller® and observed
by surgeons working under circumstances favorable for examin-
ing the question. The stimulation which excites pain in these
internal organs is usually of mechanical kind, e, g. calculus, and
the surgeon's knife and needle provide mechanical stimuli, and
Haller and his co-workers in their research employed multiform
stimuli, many of them mechanical in quality. But though
mechanical, the latter are remote in quality from the former ;
the former are distensile. The action of a calculus can be imi-
tated by injecting fluid of itself innocuous. Marked reflex
eflects can then be excited^®* from the very organs (Fig. i), the
cutting and wounding of which remains without eflect. For
Haller's and the surgical experience to be harmonized with the
medical evidence from calculi, etc., all that is necessary is that
the mechanical stimulation be adequate, and to be adequate it
must be of a certain kind. Thus we see that when the mechan-
ical stimulation employed resembles that occurring in the natural
accidents that concern medicine, the experimental results fall
into line with those observed at the bedside.

Therefore wc may infer provisionally — for the facts justify
only a guarded judgment — that the part played by the re-
ceptor in the reflex-arc is in the main what from other evidence
It is inferred to be in the case of the receptors as sense-orgdjis ;
namely, a mechanism more or less attuned to respond specially
to a certain one or ones of the agencies that act as stimuli to
the body. We may suppose this special attuning acts as does
specialization in so many cases, namely by rendering more apt
for a certain kind of stimulus and at the same time less apt for
stimuli of other kinds. The main function of the receptor is
therefore ** to lower the threshold of excitability of the art for one
kind of stimulus, and to heighten it for all others. This is quite
comparable with the low threshold for touch-sensation under
mechanical stimulation applied to a hair (v. Frey) ^"^ contrasted

1^—

I] SELECTIVE EXCITABILITY OF REFLEX-ARC 13

(v. Frey). Adaptation has evolved a mechanism for which one
kind of stimulus is the appropriate, that is, the adequate stim-
ubisi other stimuli than the adequate not being what the
adaptation fitted the mechanism for, are at a disadvantage.
Electrical stimuli are in most cases far the most convenient to
use for experimental work, because of their easy control, espe-
cially in regard to intensity and time. But electrical stimuli not
being of common occurrence in nature, there has been no chance
for adaptation to evolve in the organism receptors appropriate
for such stimuli. Therefore we may say that electricity never
constitutes the adequate stimulus for any receptor, since it is
always an artificial form of stimulus, and every adequate stimulus
must obviously be a natural form of stimulation. It is therefore
rather a matter for surprise that electrical stimuli applied to
receptor organs are as efficient excitors of reflexes as they
in fact prove to be. It is particularly in regard to a class of
reflexes whose receptive cells seem attuned specially to react to
nocuous agentS) agents that threaten to do local damage, that
electrical stimuli are found to be excellently effective. But the
conditions of adaptation to stimuli appear here peculiar ; and
there will be better opportunity of considering them later.

We infer, therefore, that the main contribution made to the
mechanism of the reflex-arc by that part of it which constitutes
the receptor is selective excitability. It thus contributes to
co-ordination, for it renders its arc prone to reply to certain
stimuli, while other arcs not having that kind of receptor do not
reply, and it renders its arc unlikely to reply to certain other
stimuli to which other arcs are likely to respond. It will thus,
while providing increase of responsiveness on the part of the
organism to the environment, tend to prevent confusion of re-
actions (inco-ordination) by limiting to particular stimuli a
particular reaction.

On the whole, we may regard the receptor as being con-
cerned with the mode of excitation rather than with the features
of conduction of the reflex-arc, and may now return to that
conduction, which itself has important co-ordinative characters.

Nervous conduction has been studied chiefly in nerve-trunks.

14 THE SIMPLE REFLEX [Lect.

Conduction in reflexes is of course for its spatially greater part
conduction along nerve-trunks, yet reflex conduction in toto
differs widely from nerve-trunk conduction.

Salient among the characteristic differences between con-
duction in nerve-trunks and in reflex-arcs respectively are the
following :

Conduction in reflex-arcs exhibits (i) slower speed as meas-
ured by the latent period between application of stimulus and
appearance of end-effect, this difference, being greater for weak
stimuli than for strong; (2) less close correspondence between
the moment of cessation of stimulus and the moment of cessation
of end-effect, 1.^., there is a marked " after-discharge; " (3) less
close correspondence between rhythm of stimulus and rhythm of
end-effect ; (4) less close correspondence between the grading of
intensity of the stimulus and the grading of intensity of the end-
effiect; (5) considerable resistance to passage of a single nerve-
impulse, but a resistance easily forced by a succession of impulses
(temporal summation) ; (6) irreversibility of direction instead of
reversibility as in nerve-trunks ; (7) fatigability in contrast with
the comparative unfatigability of nerve-trunks; (8) much greater
variability of the threshold value of stimulus than in nerve-
trunks ; (9) refractory period, " bahnung," inhibition, and shock,
in degrees unknown for nerve-trunks; (10) much greater de-
pendence on blood-circulation, oxygen (Verworn, Winterstein,
V. Baeyer, etc.); (11) much greater susceptibility to various
drugs — anaesthetics.

These differences between conduction in reflex-arcs and nerve-
trunks respectively appear referable to that part of the arc which
lies in gray matter. The constituents of gray matter over and
above those which exist also in nerve-trunks are the nerve-cell
bodies (perikarya),^*^ the fine nerve-cell branches (dendritic
and axonic nerve-fibres), and neuroglia.

Neuroglia exists in white matter as well as in gray, and there
is no good ground for attributing the above characteristics of con-
duction in reflex-arcs to that part of the arcs which consists of
white matter. It is improbable, therefore, on that ground that the
features of the conduction are due to neuroglia. Indeed there

T] CONDUCTION IN REFLEX-ARC 15

is no good evidence that neuroglia is concerned directly in
nervous conduction at all. As to perikarya (nerve-cell bodies)
the experiment of Bethe^'^ on the motor perikarya of the
ganglion of the second antenna of Carcintis, and the experiments
of Steinach ^ on the perikarya of the spinal-root ganglion, abo
the observation by Langley ** that nicotin has little effect when
applied to the spinal-root ganglion, though breaking conduction
in S]m[ipathetic ganglia, all indicate more or less directly that it
is not to the perikarya that the characteristic features of reflex-
arc conduction are referable. Similarly, the experiments of
Exner,^ and of Moore and Reynolds,**® detecting no delay in
transmission through the spinal-root ganglion, — though observa-
tions by Wundt • and by Gad and Joseph ^ had a different result,
— withdraw from the perikaryon the responsibility for another
feature characteristic of reflex-arc conduction. Again, histolog-
ical observations by Cajal, van Gehuchten, and others, indicate
that in various cases the line of conduction may run not through
the perikaryon at all, but direct from dendrite stem to axone.

As to the nerve-cell branches (dendrites, axones, and axone-
collaterals) which are so prominent as histological characters of
gray matter, they are in many cases perfectly continuous with
nerve-fibres outside whose conductive features are known by
study of nerve-trunks, and they also are themselves nerve-fibres,
though smaller in calibre than those outside. It seems therefore
scarcely justifiable to suppose that conduction along nerve-fibres
assumes in the gray matter characters so widely different from
those it possesses elsewhere as to account for the dissimilarity
between reflex-arc conduction and nerve-trunk conduction
respectively.

In this difliculty there rises forcibly to mind that not the least
fruitful of the facts which the cell-theory rests upon and brings to-
gether is the existence at the confines of the cells composing the
organism of " surfaces of separation " between the adjacent cells.
In certain syncytial cases such surfaces are not apparent, but with
most of the cells in the organism their existence is undisputed,
and they play an important rdle in a great number of physio-
logical processes. Now in addition to the structural elements

i6 THE SIMPLE REFLEX [Lect.

of gray matter specified above, there is one other which certainly
in many cases exists. The gray matter is the field of nexus
between neurone and neurone. Except in sympathetic ganglia,
the place of nexus between neurone and neurone lies nowhere
else than in gray matter. We know of no reflex-arc composed
of one single neurone only. In other words, every reflex-arc
must contain a nexus between one neurone and another. The
reflex-arc must, therefore, on the cell-theory, be expected to
include not only intracellular conduction, but intercellular con-
duction. But on the current view of the structure of the nerve-
fibres of nerve-trunks the conduction observed in nerve-trunks
is entirely and only i«/n?cellular conduction. Perhaps, therefore,
the difference between reflex-arc conduction and nerve-trunk
conduction is related to an additional element in the former,
namely, m/^fcellular conduction. If there exists any surface or
separation at the nexus between neurone and neurone, much of
what is characteristic of the conduction exhibited by the reflex-
arc might be more easily explicable. At the nexus between
cells if there be not actual confluence, there must be a surface
of separation. At the nexus between efferent neurone and the
muscle-cell, electrical organ, etc., which it innervates, it is
generally admitted that there is not actual confluence of the
two cells together, but that a surface separates them ; and a
surface of separation is physically a membrane. As regards
a number of the features enumerated above as distinguishing
reflex-arc conduction from nerve-trunk conduction, there is evi-
dence that similar features^ though not usually in such marked
extent f characterize conduction from efferent nerve-fibre to efferent
organ^ e. g., in nerve-muscle preparation, in nerve-electric-organ
preparation, etc. Here change in character of conduction is not
due to perikarya (nerve-cell bodies), for such are not present
The change may well be referable to the surface of separation
admittedly existent between efferent neurone and effector cell.

If the conductive element of the neurone be fluid, and if at
the nexus between neurone and neurone there does not exist
actual confluence of the conductive part of one cell with the
conductive part of the other, e. g. if there is not actual continuity

I] THE SYNAPSE 17

of phjrsical phase between them, there must be a surface of
separation. Even should a membrane visible to the microscope
not appear, the mere fact of non-confluence of the one with the
other unplies the existence of a surface of separation. Such
a surface might restrain diffusion, bank up osmotic pressure,
restrict the movement of ions, accumulate electric changes,
support a double electric layer, alter in shape and surface-
tension with changes in difference of potential, alter in difference
of potential with changes in surface-tension or in shape, or inter-
vene as a membrane between dilute solutions of electrolytes of
different concentration or colloidal suspensions with different
sign of charge. It would be a mechanism where nervous con-
duction, especially if predominantly physical in nature, might
have grafted upon it characters just such as those differentiating
reflex-arc conduction from nerve-trunk conduction. For in-
stance, change from reversibility of direction of conduction to
irreversibility might be referable to the membrane possessing
irreciprocal permeability. It would be natural to find in the
arc, each time it passed through gray matter, the additive intro-
duction of features of reaction such as characterize a neurone-
threshold (Goldscheider).^^ The conception of the nervous
impulse as a physical process (du Bois Reymond) rather than
a chemical, gains rather than loses plausibility from physical
chemistry. The injury-current of nerve seems comparable in
mode of production (J. S. Macdonald) ^ with the current of a
" concentration cell," a mode of energy akin to the expansion
of a gas and physical, rather than chemical, * volume-energy.'
Against the likelihood of nervous conduction being pre-emi-
nently a chemical rather than a physical process must be reck-
oned, as Macdonald well urges, its speed of propagation, its
brevity of time-relations, its freedom from perceptible tempera-
ture change, its facile excitation by mechanical means, its
facilitation by cold, etc. If it is a physical process the inter-
calation of a transverse surface of separation or membrane into
the conductor must modify the conduction, and it would do so
with results just such as we find differentiating reflex-arc con-
duction from nerve-trunk conduction.

i8 THE SIMPLE REFLEX [Lect,

As to the existence or the non-existence of a surface of
separation or membrane between neurone and neurone, that is
a structural question on which histology might be competent to
give valuable information. In certain cases, especially in In-
vertebrata, observation (Apathy, Bethe, etc.) indicates that many
nerve-cells are actually continuous one with another. It is note-
worthy that in several of these cases the irreversibility of direc-
tion of conduction which is characteristic of spinal reflex-arcs is
not demonstrable ; thus the nerve-net in some cases, e.g. Medusa,
exhibits reversible conduction (Romanes, Nagel, Bethe, and
others). But in the neurone-chains of the gray-centred system
of vertebrates histology on the whole furnishes evidence that a
surface of separation does exist between neurone and neurone.
And the evidence of Wallerian secondary degeneration is clear
in showing that that process observes strictly a boundary
between neurone and neurone and does not transgress it. It
seems therefore likely that the nexus between neurone and neu-
rone in the reflex-arc, at least in the spinal arc of the vertebrate,
involves a surface of separation between neurone and neurone ;
and this as a transverse membrane across the conductor must
be an important element in intercellular conduction. The
characters distinguishing reflex-arc conduction from nerve-
trunk conduction may therefore be largely due to intercellular
barriers, delicate transverse membranes, in the former.

In view, therefore, of the probable importance physiologically
of this mode of nexus between neurone and neurone it is con-
venient to have a term for it The term introduced has been
synapsey'^

The differences between nerve-trunk conduction and reflex-
arc conduction are so great as to require for their exhibition no
very minute determination of the characters of either ; but we
may with advantage follow these differences somewhat further.
In doing so we may take the reflexes of the hind limb of the
spinal dog as a field of exemplification.

Reflex latency. A dissimilarity between nerve-trunk con-
duction and reflex-arc conduction which has often been stressed
is the slowness of the latter as measured by the latent interval

I] LONG LATENCY OF WEAK REFLEXES 19

between application of stimulus and appearance of end-effect
In nerve-trunks the interval between the moment of stimulation
and the appearance of response (electrical) at any distant point
is strictly proportional to the distance of that point from the seat
of stimulation. There is in the nerve-trunk no measurable delay
or latent interval for the response at the seat of excitation.
TTie latent time for nerve-trunk response is therefore entirely a
propagation time. The speed of propagation in frog's nerve
at 15** C. is about 3 cm. per sig^a (cr = .ooi second). We
may compare with this the latent period of the flexion-reflex
of the " spinal " dog's hind leg. The movement of this reflex
is a flexion at knee, hip, and ankle. It is easily and regularly
evoked by nocuous or electrical stimuli applied to the skin of
the limb or to any afferent nerve of the limb. For measure-
ments of the reflex latency I have stimulated with break or
make shocks of regular but varied frequency. Assuming that
in warm-blooded nerves the conduction is the same (Helmholtz
found it faster) as in the frog, and that the length of the reflex-
arc of the dog's knee is two thirds of a metre, and assuming
that we may add 5 o- for mechanical latency of the flexor con-
traction of the limb, we should have about 27 cr as the latent
time for the flexion-reflex, supposing its conduction proceeded
as does nerve-trunk conduction. But, as a fact, a period double
that is common enough for this reflex under ordinary moderate
intensities of stimulation.

But with intenser stimuli the latent period of this reflex is
much less. A period of 300- from commencement of stimu-
lus to commencement of mechanical response is not then un-
common. I have met, at shortest, with 22 a. There is here
little difference between speed of reflex conduction and speed
of nerve-trunk conduction. Similarly Francois Franck"^ has
recorded latent periods for reflex action differing little from
those of simple nerve-trunk conduction. Thus, 1 7 a were ob-
tained for a reflex contraction of the crossed gastrocnemius
evoked by stimulation of the afferent root of the first lumbar
nerve. These short latencies Franck obtained with strong
stimuli

It would seem, therefore, that the more intense the stimulation

:.wt .M.cr.Ai LIBFIARY «

I] LONG LATENCY OF WEAK REFLEXES 21

the more the conduction along the reflex-arc comes to resemble
in speed the conduction along simple nerve-trunks.

It is with mild stimuli that the difference in speed between
reflex conduction and nerve-trunk conduction becomes most
obvious. The latent period for the flexion-reflex, then, lies
usually between 600- and 1200-. I have met with it as long as
2000-. There is no good evidence that the speed of propaga-
tion in nerve-tnmk conduction is in response to weak stimuli
appreciably slower than to strong. This slackening of propa-
gation speed under weak stimuli (Fig. 2) is, I would urge, a more
significant difference between reflex-conduction and nerve-trunk
conduction than is the mere greater slowness of the former than
of the latter. Another difference between the two in regard to
conduction-speed is that in the various cerebrospinal nerve-
trunks of the same animal species the conduction-speed ap-
pears to be practically the same. But reflex conduction-speed
as measured by the latent period differs greatly in the various
type-reflexes of even one and the same limb. The latent time
of the scratch-reflex is, on the average, much longer than that
of the flexion-reflex or extensor-thrust, although the spatial dis-
tance of the nerve-fibre conduction is not greater. The latency
of the former usually in my experience lies between 140 a for
intenser stimulation and 500 a for weaker, and I have seen it ex-
tend to 24400- and even to 35400-. So that although a weakly
provoked flexion-reflex may have a lengthier latency than a
strongly provoked scratch-reflex, the latency of the scratch-
reflex is nevertheless on the average very characteristically
longer than that of the flexion-reflex. Now there is no evidence
that this is referable to a difference in the conduction rate along
the nerve-trunks of the two reflexes ; indeed, the efferent nerve-
trunks for the two reflexes are the same.

The speed of travel of nervous impulses along nerve-trunks
is fairly known. On the not improbable assumption that their
velocity along the myelinate fibres of the white tracts of the
central nervous system is about the same as along the myelinate
fibres of nerve-trunks, the latent period of reflex-actions of
moderate intensity is obviously greater than can be accounted

22 THE SIMPLE REFLEX [Lect.

for by travel along such conductors of the same length as the
reflex-arc itself. The delay in speed occurs whenever the
impulses pass through gray matter. This has been especially
clearly shown by Exner.^*' The delay in the gray matter may
conceivably be due to slower conduction in the minute, branched,
and more diffuse conducting elements — perikaiya, dendrites,
arborizations, etc. — found there; or it may be referable to a
fresh kind of transmission coming in there, a process of trans-
mission different in nature to conduction along nerve-fibres.
The neurone itself is visibly a continuum from end to end, but
continuity, as said above, fails to be demonstrable where neurone
meets neurone — at the synapse. There a different kind of trans-
mission may occur. The delay in the gray matter may be refer-
able, therefore, to the transmission at the synapse.

And if the delay occur at the synapse, the possibility sug-
gests itself that the time consumed in the latent period may
be spent mainly in establishing active connection along the
nervous-arc, which connection once established, the conduction
in the arc then proceeds perhaps as speedily as does conduction
in a simple nerve-trunk. The latent time would then be com-
parable with time spent in closing a key to complete an electric
circuit or in setting points at a railway junction. The key once
closed, the points once set, the transmission is as expeditious
there as elsewhere. Measurements of reflex times deal custom-
arily, so far as I am aware, with the latent time of reflexes initiated
in arcs fiot in action at the moment when the exciting stimulus is
applied to their afferent end. How the latent time is spent can
receive some light from observation on the latent time of an
increase of action in an arc already active in the same direction
as the incremental action.

To examine this the flexion-reflex was excited by a sub-
maximal stimulus, and after its appearance the intensity of the
exciting stimulus was abruptly increased by short-circuiting a
definite resistance from the primary circuit The stimulus was
a series of break shocks of regular interval given by a key
rotating at constant speed in the primary circuit of the induc-
torium. An electromagnet marked the interruptions of the

24 THE SIMPLE REFLEX [Lect.

primary current ; the electromagnet was arranged to show by
more ample excursion of its armature the point of time firom
which onward the primary current was increased. The shocks
were applied by a needle-point (kathode) to the skin of a digit:
the other electrode, large and diffuse, was wrapped round a fore-
foot, I. /., headward of the spinal transection. In these experi-
ments the earlier reflex elicited may be termed the imiial ritJUx.
Its sudden increase on sudden intensification of the stimulus
may be termed the incremental reflex. The latent times of the
initial reflex and incremental reflex, when compared^ showed
almost always that the latency of the latter was rather die
shorter. But the difference often was not great (Figs. 3 and 4).
The average for 30 of the initial reflexes was 48 o-, and for the
30 corresponding incremental reflexes was 38 <r. This difference
seems too small to support the supposition that the latent time
of the initial reflex is chiefly consumed by" setting "the synapse,
which, once set, conducts in much the same way as regards speed
of transmission as does the rest of the arc. It might be that the
incremental reaction involved the " setting " of other additional
synapses. But such an explanation demands that none of the
augmentation occur through the synapses already in action, for
the latent time is measured to the flrst beginning of the steplike
incremental ascent of the curve*

In the incremental stage of the reaction the reflex is usually
a relatively intense one. Now the length of reflex latency is
caeteris paribus inversely as the intensity of the reflex. If the
reflex as produced in two stages be compared with the reflex
produced by delivery of the stimulus in its full strength at the
outset, the latent time of this latter is found shorter than that of
either the initial or incremental reaction of the other reflex
(Fig. 4). The latent time under the same external stimulus is
thus less in circumstances that ex hypothesi involve building a
bridge and then sending impulses across it, than when the bridge
already having been built the impulses have merely to pass.
This argues against an amoeboid movement of the protoplasm
of the cell being the step which determines its conductive com-
munication with the next (Demoor, Cajal, Renaut, Monti, Duval,

FiGUiui 4. — Satne as preceding (Fig. 3), but with tomewhat stronger initial ftimuluA and
rtftcx. At the extreme right hand of the figure is shown the beginning of the " total " re*
Seait that is, the reflex when the intensity of stimulus used incrementaUy in the Ifift-hand
obMrraticm is thrown in at outset. There is little difference between the latent times of
the "initial** and ** incremental *' reflexes; the latent time of the "total" reflex is
iliorteT than that of cither the initial or incremental reflex. Electromagnet above records
break shocks as before ; at A the initial^ at B the incrgmtntat^ at ^ the toiai stimulus.
The time below is given in j\^ sec* and in seconds.

Lugaro). It also scemsconclusi ve against any major portion of the
latent period being consumed at the synapse in a process which
sets the synapse ready to conduct, — a process of preparation
for transmission as distinguished froora process of transmission.
It argues that the delay is inherent in the process of transmis-
ston itself* and that therefore the actual nervous transmissioa
at these points has, when the stimuli are weak, a differetit order
of speed to that in nerve^fibres. The shorter latent time of the
reflex induced by the stimulus delivered in full at the outset is in

26 THE SIMPLE REFLEX [Lect.

harmony with the reflex, being under that mode of excitation
rather more intense (/. ^., of greater amplitude) than when
excited as an increment to a foregoing reflex of less strength.
The shorter latent times given by intenser stimuli seem readily
explicable by the minimal quantity of transmitted influence nec-
essary to give detectible eflect, being necessarily earlier reached
with copious transmission than with weaker transmission.

The observations indicate, therefore, that the latent time
belongs to some process which is the same in nature, both in
initiating a reflex from a resting arc and in increasing a reflex
through an arc already in submaximal activity, — and probably
therefore in maintaining a reflex in unaltered continuance in an
arc. It argues that any " setting " process in the nerve-centre,
if it occur at all, is negligible in regard to the time it consumes.
It suggests that even while at rest the reflex apparatus is just
as prepared for immediately transmitting impulses as when
actually engaged in reflex activity in the very direction the new
impulses would require. It therefore suggests thie greater need
for active inhibition in the co-ordination of activity of arcs which
have a final path in common and yet use that path to different
eflects. If resting paths all lie open for conduction, prevention
of confusion must depend not on the path excited being the
only one open for conduction, but on its excitation being accom-
panied by inhibition of others that, did they enter into action,
would detrimentally confuse the issue of events.

Reflex after-discharge. Another characteristic diflerence be-
tween conduction in nerve-trunks and in reflex-arcs is the less
close correspondence in the latter between moment of cessation
of stimulus and moment of cessation of end-effect. The reflex-
arc shows marked " after-discharge " ; the nerve-trunk does not.
Tetanic contraction of the knee-flexor muscles of the dog in-
duced by brief faradization of the motor-nerve usually ceases
within 1500- of the cessation of the stimulation of the nerve,
if crude condition of fatigue, etc., be avoided. The contraction
of those same muscles, when induced reflexly by a similar brief
stimulation, often persists for 50000- after cessation of the stimu-
lus (Fig. 5).

28 THE SIMPLE REFLEX [Lect.

InUntUy 0/Mtimmlus, Mmtwrt o/rtJUx, MtatMrt o/a/Ur^diackmrgw*
I. 69 110 50

II. no 373 161

III. 190 782 6j6

IV. 300 1 196 1016

Time in seconds below. Clonus is seen in the af ^^er-disdiarge. The meume of Intmsitj
of stimulus is given from the units of the Kronecker coil; the meMore of tiio reflex is
from the area included between myograph curve and base line. Keflex IV not in tiio figure.

We must subtract from the period of after-discharge a period
equal to the latent time. But usually the latent time is quite
insignificant in length as compared with the after-discharge.
The after-discharge in flexion-reflex VI (Fig. 6) was more than
Ave hundred times longer than the latent time.

The after-discharge increases with increaseof intensity of the
stimulus, not only absolutely, but relatively to the whole reflex —
when the stimulation is not long lasting. Taking the flexion-
reflex for example, the increase of after-discharge with increas-
ing intensity of stimulation is more marked than the increase of
contraction-height (Fig. 7). With stimulation lasting not more
than 1000 <r the maximum amplitude of the reflex arrives, as the
intensity of the stimulus is increased, later and later, so that with
weaker stimuli it falls within the excitation period, but with
stronger stimuli it is reached only after the application of the
stimulus has ceased. Very marked in the after-discharge of
this, the flexion-reflex, is a clonus (Figs. 5, 6) with a rate in my
records varying between 7.5 and 12 per second. Undulations
of similar kinds are clear in the reflex movement even from
the outset in weak intensities of the reflex.

FiGU RE 6. — Flexion-reflex. Stimulus for each reflex was 72 break shocks at the rata of 40
per second. This stimulus is registered above by electromagnet in the primary. Ab-
scissae on the myograph curve show its delivery and cessation on that cunre. The cnnre
marked V is the actual record of its observation ; the other curves are tiadngs of the
records obtained in the consecutive series of which curve V was the fifth member; the
record is thus condensed and contrasted.

119

509

693

964

143a

The clonus of the after-discharge is well seen. Time below in seconds.

InitHsity ofstimtdut.

Metuur* o/reJUx.

350

II.

475

aoo

III.

690

666

IV.

1100

913

1900

1277

VI.

3000

1765

VII.

350

30 THE SIMPLE REFLEX [Lect.

Under short-lasting application of rather weak serial stimuli
at slow frequency of repetition (break shocks at 20 per second),
increase of the number of stimuli, without alteration of their in-
tensity or rate, in other words mere prolongation of the stimula-
tion, increases the after-discharge (Fig. 8). The reflex induced
by nine stimuli has an after-discharge three times as great as the
reflex from three similar stimuli. The maximum amplitude in
this case may remain practically the same, the later stimuli
simply prolonging the maximum without increasing it. They
prolong it for a far greater time than their own delivery pro-
longs the stimulation time.

In the scratch-reflex, likewise, intensity increases -the after-
discharge (Fig. 9). Its after-discharge is rhythmic, a clonus
like the rest of the reflex, with the slight lengthening of duration
and sequence of the terminal beats that is characteristic of this
reflex. The after-discharge from the scratch-reflex is not
usually so prolonged as that from a flexion-reflex produced by
a stimulus of like length and intensity (Fig. 10). Six to nine
beats usually complete the after-discharge.

In the spinal dog there is a reflex of the hind limb in which
a movement of extension at knee, ankle, and hip is caused
by stimulation of the skin of the contralateral hind limb — the
** crossed extension-reflex." When this reflex is provoked wth
more than a certain intensity its after-discharge becomes a
feature of extraordinary prominence, both as regards degree of
contraction and duration. This after-discharge may then be
more intense than any other part of the reflex, and may persist,
gradually declining for 10 or 15 seconds (Fig. 27). Wundt*
and Biedermann ^o* have noted that in the cooled frog the dura-
tion of the reflex after-discharge is prolonged.

FiGURK 7. — Flexion-reflex. Elicited by lo break shocks at rate of 20 per leoond, /.«.,
stimulation lasting .5". Intensity of stimuli increased by bringing secondary ooil towud
primary*.

Stimmlus. fVA^rtjlex. After^uekargt^

Top reflex 30 12 9

Next •» 45 52 43

Next " 65 130 118

Next " 85 176 158

Bottom reflex loo 238 236
The intensity of stimulus is given in units of the Kronecker inductorium. The amount

of reflex is measured by the area between the myograph curve and the base line.

AFTER-DISCHARGE

bdnctodttm the break shocks of whidi were delivered through a needle electrode (katb*
o(k> to til* pUntar skin of the outermost digit. An adjtistible spring rheotome allowed
ehe desired Qomber d intemtptioiis tn the primary, and therefore the desired number of
bicak shocks in the exdtiiig orcnit when that wa^ unshortcircuited. Three of the suc-
ccair?e stimali ehdted the uppernost reflex, four the neitt^ five the next, six the next,
aad mine the lowest. The '* after-discharge " is seen to be increased by mere prolonga-
tioo of the stimulus within these limits. The frequenqr of the stlmuU rcmaiaed ta &U
fliTi SO per second, and their mtensity was the same for all the reflexes,

150 r ao

*»' 3?

^50* $3

Time in seconds above lowest record.

»'*.^'.i

FicunB 9- — Effect of intensity of stimulus on scratch-reflex. Stimulus is 9 break shocks at
cue of 25 per second delivered to a point in the scapular skin by unipolar faradization,
the sitgmatic electrode being the kathode. A, the stimulus is very weak : one small beat
of clianctcristic slowness is evoked after a long latent period; B, increase in intensity o(
shoda with resulting shorter latent time and a reHex movement of two feeble beats ;
Cf further increase of intensity of stimulus : the latent time b shorter^ and a reHex of ten
tia\y quick and ample beats ensues. The stimulus lasted less than a half second ; the
reflex is not completed for more than two seconds after cessation of the stimulus.

There is no feature of the conduction of a reflex-arc which
distinguishes its mechanism more universally from that of a
mere nerve-fibre tract or trunk than lengthy after-discharge.
Richet*^ has paradoxically applied to this feature the old
adage modified: ** Sublata causa, non tollitur effectus/* The

THE SIMPLE REFLEX

after-discharge can, however, be cut short sharply by " inAibu
ticn**; it seems also to remanifest itself sometimes after a
passing interruption by inhibition.

The long latency and the marked after-discharge of reflex-
conduction easily explain a phenomenon often met when study-
ing reflexes provoked by stimuli that are brief, especially if they

Figure io» — Scratch-reflex and flenion-rcflet provoked by similar stimulation In the same
animal in quick succession. Stimulattoo was unipolar faradization with 45 break shocks;
the kathode was stigmatic and applied^ to the fhoulder skin for scratch>reflex, to the
fourth toe for flexion-reflex; diffuse electrode on forefoot. Frequenqr of shocks, 18
per lecond. The after-discharge of the scratch-reflex Lasted barely one second ; that of
the flexion, nearly eight seconds. Time is marked in seconds below each record ; above,
an electromagnet in the primary circuit records the interruptions giving the shocks.

be also weak. The stimulus, though it may last for a good
many si^mata, is over and past ere the reflex-response appears
(F*g- 9)' That response, when it appears, may nevertheless
endure for TOGO <r or more. There is nothing closely compar-
able with this in the conduction of nerve-trunks.

The after-discharge of a reflex may be considered analogous
to Sl positive after-image left by a visual stimulus. The analog

I] AFTER-DISCHARGE 35

is suggestive in connection with others to be drawn between
spinal and visual phenomena*

Conduction along reflex-arcs presents in contrast to that
along nerve-trunks characters that may be figuratively described
as indicating inertia and momentum. It is as though in the
case of a weight to be pulled from a position of rest the tractive
force were applied through a rigid rod in nerve-trunk conduc-
tion, but through a relatively 3rielding elastic band in reflex-arc
conduction. But there are other diflerences between the two
forms of conduction which this simple simile does not flgure.
We have to enter on such at our next meeting.

36 THE SIMPLE REFLEX [Lect.

LECTURE II

CO-ORDINATION IN THE SIMPLE REFLEX {continued)

Argument: Reflex-arcs show high capacity for summing excitations.
Irreversibility of direction of conduction in reflex-arcs. Reversibility
of direction of conduction in certain nerve-nets, e, g. that of Medusa.
Independence between the rhythm of the reflex-discharge and the
rhythm of the external stimulus exciting it Refractory phase in re-
flexes; in the eyelid-reflex; in the scratch-reflex. The neuronic
construction of the reflex-arc of the scratch-reflex. Long descend-
ing propruhspinal tracts revealed by the method of ''successive
degeneration." The ** final common path " and the " aflerent arc."
Intraspinal seat of the refractory phase of the scratch-reflex. The
value of refractory phase in the co-ordination of the swimming of
Medusa. Its value in the co-ordination of the scratch-reflex. Signifi-
cance of the intraspinal situation of the refiractory phase of the scratch-
reflex. Other instances of " central " refractory phase.

Bommation. Summation of subliminal stimuli so that by repe-
tition they become cflfective is practically unknown in nerve-
trunk conduction. But it is a marked feature of reflex-arc
conduction (Setschenow,*^ Stirling*^). Nor is it attributable to
the muscles whose contraction may serve as index of the reflex-
response, since summation of this extent is not known for
vertebrate skeletal muscle, though found by Richet* in the
claw-muscle of the crayfish.

We find striking instances of the summation of subliminal
stimuli given by the scratch-reflex. The difficulty in excit-
ing a reflex by a single-induction shock is well known. A
scratch-reflex cannot in my experience be elicited by a single-
induction shock, or even by two shocks, unless as physiological
stimuli they are very intense and delivered less than 600 <r apart.
Although the strongest single-induction shock is therefore by
itself a subminimal stimulus for this reflex, the summating power
of this reflex mechanism is great Very feeble shocks, each

U] SUMMATION 37

SDCceeding the other within a certain time — sununation time —
sum as stimuli and provoke a reflex. Thus long series of sub-
minimal stimuli ultimately provoke the reflex. I have records
where the reflex appeared only after delivery of the fortieth suc-
cessive double shock, the shocks having followed each other at a
frequency of 1 1.3 per second, and where the reflex appeared only
after delivery of the forty-fourth successive make shock, the shocks
having followed at 18 per second. A momentary stimulus, e.g-., a
break shock of fair physiological strength applied by a stigpnatic
pole (needle point) to a skin-spot in the receptive field of thi^
reflex, produces in the nervous arc a change which though, as
just said, unable of itself alone to produce the reflex movement,
shows its facilitating influence {bahnung) on a subsequent stimu-
lus applied even 1400 a later. The duration of the excitatory
change induced by a momentary stimulus is therefore in this
mammalian arc (scratch-reflex) almost as long as that noted in
the frog by Stirling, namely, 1500 o-.

With serial stimuli of the same frequency of repetition the
latent time of the scratch-reflex is shorter the more intense
the individual stimuli. Stirling ^ conclusively traced length of
latency to dependence on spinal summation of successive excita-
tions. In accord with this in the "scratch-reflex," when the
serial stimuli follow slowly, the reflex caeteris paribus is pro-
longed. A single brief mechanical stimulation of the skin (rub,
prick, or pull upon a hair) usually succeeds in exciting a scratch-
reflex, though the reflex thus evoked is short ; but there is noth-
ing to show that these stimuli, though brief, are really simple
and not essentially multiple. A striking dissimilarity, therefore,
between reflex-arc conduction and nerve-trunk conduction is
that in reflex-arc conduction considerable resistance is oflered
to the passage of a single nerve-impulse, but the resistance is
easily forced by a succession of impulses ; in other words, sub-
liminal stimuli are summed.

It follows almost as a corollary from this that the threshold
excitability of a reflex mechanism appears much more variable
than tiiat of a nerve-trunk, if the threshold excitability be meas-
ored in terms of the intensity of the liminal stimulus. The value

38 THE SIMPLE REFLEX [Lect.

will be more variable in the case of the reflex mechanism, be-
cause there the duration of the stimulus is a factor in its effi-
ciency hr more than in the case of the nerve-trunk. In the
scratch-reflex a single stimulus which is far below threshold
intensity is found, on its fortieth repetition and nearly four
seconds after its first appplication, to become effective and pro-
voke the reflex.

Irrereisfbility of direction of oondnotion. Another remarkable
difference between reflex-arc conduction and nerve-trunk con-
duction is the irreversibility of direction of the former and the
reversibility of the latter. Double conduction, as it has been
termed, is well-established for nerve-trunks both afferent and
efferent It was shown by du Bois Reymond for the spinal
nerve roots, for peripheral nerves by Kuhne's gracilis experi-
ment, for the great single electric fibre of Malapterurus by
Babuchin,^ for sympathetic nerve-cords by Langley and Ander-
son,^ and by myself *^^ for certain fibres of the white tracts of
the spinal cord. The nerve-fibres in all these. cases, when ex-
cited anywhere in their course, conduct nerve-impulses in all
directions from the point stimulated ; that is, in their case both
up and down, the only two directions open to them. Their
substance may therefore be regarded as conductive in all direc-
tions along their extension.

From the Bell-Magendie law of the spinal nerve-roots we
know that reflex-arcs conduct only in one direction. The
stimulation of the central end of a motor-nerve remains without
obvious effect Bell ^^ and Magendie *^ and their followers estab-
lished that excitation of the spinal end of the severed motor root
evokes no sign of reflex action or sensation. Evidendy the
central nexus between afferent channel and efferent is of a kind
that, though it allows conduction from afferent to efferent, does
not allow it from efferent to afferent The path is patent in one
direction only. This is the special case which forms the first
foundation of the law that conduction in the neural system pro-
ceeds in one direction only, the " law of forward direction (W.
James, 1880).® When the property of double conduction in
nerve-fibres had been ascertained, the Bell-Magendie law of the

II] IRREVERSIBLE CONDUCTION 39

spinal roots became more instructive. Gad^ (1884) ai^ed
that the dendrites of the motor root-cell are capable of conduc-
tion in one direction only, namely, toward and not away from
the axone. It may, however, be that the irreciprocity of the
conduction is referable to the synapse. The explanation of the
valved condition of the reflex circuit may lie in a synaptic mem-
brane more permeable in one direction than in the other. In
other words, through intraneuronic conduction is reversible
in direction, interneuronic may be irreversible.

Cell-chains of polarized conduction form the basis of the
great majority of all the nervous reactions of the cerebrospinal
system of higher animals. It appears, however, that not all
pluricellular nervous circuits exhibit irreversible direction of
conduction. The nerve-net of Medusa is a pluricellular con-
ductor which exhibits reversibility of direction of conduction.
In Medusa locomotion is effected by contraction of a sheet of
muscle in the swimming-belL When the swimming-bell, which
resembles an inverted cup, contracts, its capacity is lessened,
and some of the water embraced by it is expelled through the
open end, the animal itself being propelled in the reverse direc-
tion by recoil. The mechanism is like that of the heart, but
the heart propels its contents, the swimming-bell propels, itself
against its contents. The contractions of both recur rythmi-
cally, though Medusa, unlike the heart, has periods of prolonged
diastolic inactivity. At such a period an appropriate stimulus
restarts the swimming-bell. The contractive beat begins from
the point stimulated and spreads thence over the whole muscu-
lar sheet^ It spreads rapidly enough for the contraction not
to have culminated at the initial point before it has set in at the
most remote part The beat is thus not only everywhere in
progress at the same time, but is practically in the same phase of
progress everywhere, and similarly synchronously passes off.^

The arrangement of the nervous system of Medusa, e. g.,
Rkizostoma^ is, according to Bethe, of the following kind
(Fig. II). The nerve-cell has on one hand thread-like arms
that extend to the surface of the subumbrella, and on the other
hand others which stretch down to the sheet of contractile cells

THE SIMPLE REFLEX

[Lect.

'#»

I Mi f '

:mi

♦»

^i.f

/^, ^

kpL

FiGUiiB II (A. Bethc ««), — Nenre^oH of Rhizoitoma. A, radUI section through i mus-
cular field of the subumbrella ; £/, epitbeliuin ; m, musde-ftbr^ in cross^ecttoo; M* JT.,
their nuclei ; A^.//, nerve-plexus with fibres runiung into the epithelium and to Chetniitdes;
0^ nerve-pl«aus with scattered cells, from a horizontal section, Ma^ificatioti ixoo in A» n
soo in B. I

on the under side of the bell. Each nerve-celt has also long
side threads which join similar side threads from other nerve-
cells* By virtue of these lateral connections the nerve-cells form
a network of conductors spreading horizontally through the bell
in a layer of tissue between a receptive sheet and a contractile
sheet From this nerve-net, throughout its extent, there pass
nerve-threads to the adjacent muscle ; it also receives at many
points of its extent nerve-threads from specially receptive areas
of surface.

nj IRREVERSIBLE CONDUCTION AT SYNAPSE 41

The circularly arranged sheet of muscle does not form a
continuous field toward the centre of the disc ; there are wide
radial gaps in it. Across these gaps the " conduction " passes :
the microscope reveals no muscular tissue in these gaps, but the
nerve-net can be seen to spread across them.*^ The presence
of the nerve-net explains the conduction across them. It is
therefore ai^ed by Bethe that the spread of the contraction
over the muscular sheet in Rhizostoma does not imply conduc-
tion of the contraction fi-om one muscle-cell to another, but is
the result of the spread of nervous action over the nerve-net
work. In its progress along the nerve-net, the nervous discharge,
as it reaches each part of the nerve-net, spreads down the nerve-
threads, descending thence to the underlying muscle-sheet. So
long as the nerve-cell network is intact, wherever the point
stimulated, the ensuing contraction is of the whole bell, that is,
the nerve-impulses started at one point of the receptive surface,
on entering the nerve network, spread over it in all directions.
When the bell-shaped disc is spirally cut into a long band, to
whichever end of the band the stimulus be applied, the conduc-
tion spreads from that end to the other and over the whole
strip (Romanes)." The nerve-net therefore conducts nerve-
impulses in both directions along its length. Therefore it is not
a polarized conductor, conductive in one direction only. In the
chains of nerve-cells of higher animals, such as Arthropods and
Vertebrates, although the conduction is reversible in each nerve-
cell, — at least along that piece of it which forms a nerve-fibre, —
the pluricellular chain in toto constitutes a polarized conductor,
conductive in one direction only. In such cell-chains the indi-
vidual nerve-cells are characterized morphologically by possess-
ing two kinds of cell-branches, which differ one from another
in microscopic form, the one kind dendrites, the other axones.
The difference in appearance between dendrites and axones is
marked enough for recognition by microscopical inspection.
Since in many well-known instances the dentrites conduct im-
pulses away from their free ends, while the axone conducts
towards its free end, it is possible on mere microscopic inspection
of nerve-cells of this type to infer by analogy the normal direction

42 THE SIMPLE REFLEX [Lect.

of the conduction through the nerve-celL But in the nerve-cells
forming the nerve-network of Medusa there seems no such dis-
tinct differentiation of their branches into two types. Their cell-
processes are not distinguishable into dendrites and axones.

Moreover, microscopic examination of the nerve-net of
Medusa reveals another difference between it and the nerve-cell
chains of higher animals. In these latter the neuro-fibrils of one
nerve-cell are not found unbrokenly continuous with those of
the next cell along the nerve-chain. Although the union may
be close, there is not homogeneous continuity. The one nerve-
cell joins another by synapsis. But in the nerve-net of Medusa
the neuro-fibrils pass, according to Bethe, uninterruptedly across
from one cell to another. Even if we admit the neuro-fibrils to
be in a measure artifacts, the appearance of their continuity
from one cell to another in one type, and of their discontinuity
from one cell to another in the other type remains significant of
a difference between the conduction-process from cell to cell in
the two types. The nerve-net of Medusa appears an unbroken
retiform continuum from end to end. Each nerve-cell in it
joins its neighbours much as at a node in the myelinate nerve-
fibre the axis-cylinder of each segment joins the next Reversi-
bility of conduction may be related to this apparent continuity
of structure, and irreversibility to want of it This points to the
latter's being referable to the synapse ; if the synaptic membrane
(Lect. I. p. 1 8) be permeable only in one direction to certain
ions, that may explain the irreversibility of conduction. The
polarised conduction of nerve-arcs would be related to the
one-sided permeability of the intestinal wall, e.g, to NaCl
(O. Cohnheim).

Rhythm of reBponse. One of the differences between nerve-
trunk conduction and reflex-arc conduction is the less close
correspondence in the latter between rhythm of stimulus and
rhythm of end-effect. The number of separable excitatory
states (impulses) engendered in a nerve-trunk by serially re-
peated stimuli corresponds closely with the stimuli in number
and rhythm. Whether the stimuli follow each other once per
second or five hundred times per second, the nervous responses

II] RHYTHM OF END-EFFECT 43

follow the rhythm of stimulation. Using contraction of skeletal
muscle as index of the response the correspondence at rhythms
above thirty per second becomes difficult to trace, because the
mechanical effects tend at rates beyond that to fuse indistin-
guishably. The electrical responses of the muscle can with ease
be observed isolatedly up to faster rates : their rhythm is found
to agree with that of stimulation ; thus, at eighty per second
their responses are eighty per second If the muscle note be
accepted as an indication of the response of the muscle, its pitch
follows /tfri passu the rate of stimulation of the nerve through
a still greater range.

The case is quite different with reflex-arcs. Schafer ** noted
undulations of a frequency of ten to twelve per second on myo-
grams of spinal reflexes evoked by excitation of the afferent
nerve by faradic currents of frequency much above ten to twelve
per second. In such a case we may assume the absence in the
afferent nerve itself of any refractory period long enough to give
a ten per second rhythm to the response. The refractory period
in nerve-trunk conduction seems to last not longer than i a.
The rhythm of discharge from the motor-cell, as far as the un-
dulations noted indicate rhythmic response, are totally different
in rhythm from that of the action induced in the afferent cell by
the stimulation applied. In the reflex centre the rhythm has
been transmuted from one rate to another. Schafer refers this
change to the synapse.

Again, as noted above, undulations at rates varying be-
tween 7.5 and 12 per second are seen in the flexion-reflex both
in its after-discharge and during the excitation and quite inde-
pendendy of the rate of delivery of the induction shocks used
as stimuli (Fig^s. 6 and 12), and even when the stimulus is a
constant current Again, reflexes of weak intensity, both in the
case of the flexion-reflex and of the crossed extension-reflex,
exhibit at their commencement a stepped form of myogram:
the steps usually succeed each other at about eight per second,
and their rate is independent of the rate of delivery of the electric
stimuli (Fig. 12) exciting the reflex.

In such cases, therefore, the rhythm of the end-effect indi-

FiGUHB f 2. ^ FleEion*nfl€ic showing imperfect fusion under exdution of the skin with
break shocks at the rate of ro per second. Signal above ^ows thesis ioterruptioiis of
the primary circuit. Time below in seconds.

cates that in transmtssion along the reflex-arc the impulses
generated at the receptive end of the arc are not actually passed
on from one cell element to another in the arc, but that new
impulses with a different period are generated in the course of
the reflex-conduction. This is confirmatory of a neurone-
threshold as a feature in central conduction.

Refractory phase. A conductor which replies intermittently
to a stimulus exhibits a refractory phase. It seems that even
in nerve-trunk conduction a refractory phase must occur; other-
wise, the conductor being capable of reversible direction of con-
duction, a backward propagation of the excited state as well
as a forward would ensue from every point of the conductor
reached by the nervous impulse. The excited state would then,
when once excited » maintain itself in a tetanic manner along the
whole length of the conductor. The propagation would thus
lose undulatory character such as we know it has; it would
merely have an initial wave-front. But this refractory phase in
nerve-fibres seems of very brief duration, not longer than i «r.

n] REFRACTORY PHASE 45

The reflex-discharge from nerve-cells seems to oe rhythmic
even under continuous stimulation ; but the cases in which this
has been examined are sparse. The existence of rhythm in
nerve-cell discharge is presumptive evidence of a refractory
phase in their reaction. Refractory phase was first called atten-
tion to by Kronecker and Stirling" in 1874, in the heart, and
recognized by them as a fact of central importance for cardiac
rhythm. In 1876 Marey '^ met the same phenomenon and gave
it the name by which it is now known. A year later Romanes'
fundamental work on Medusa demonstrated the existence of the
same phenomenon there.^ The inconspicuous duration of the
phase in nerve-trunk conduction and the progress of the view that
regards the heart beat as of myogenic origin have contributed
to delay recognition of refractory phase as a character of reflex-
arc reactions. But in 1899 Zwaardemaker and Lans^^ showed
the supervention of a refractory phase in reflex eyelid-closure.
By refractory period was originally meant by Marey the time
during which the heart was inexcitable to a stimulus however
intense. But to-day by refractory phase is understood a state
during which, apart from fatigue, the mechanism shows less than
its full excitability. The cardiac refractory phase is absolute
for a short time after commencement of systole, but excitability
then returns gradually. In the eyelid-reflex for nearly a full
second after initiation of a reflex, the chance that a second
stimulus then delivered will, though otherwise appropriate, ex-
cite the reflex, is flfty per cent less than it is one second later.
The refractory phase, therefore, is marked though not absolute :
it operates longer for a visual stimulus than for a tactual or
thermaL

Of the spinal reflexes of the dog's hind limb, one, namely, the
scratch-reflex, shows marked refractory phase.*^ When work-
ing originally with this reflex, I noticed ^* that the rhythm of
the scratching movement is in rate independent of the rhythm
of the stimulus evoking it

In the dog, when the spinal cord has been transected in the
neck, the scratch or scalptor reflex becomes in a few months
prominent. A stimulus applied at any point within a large

•^w: M.L! /». M^Ain Of
BMLO 4LTiJ, CAUfOWlA

THE SIMPLE REFLEX

[Lect.

'rer

F^GURK tv — A« The '^ receptive field,*' as revealed after low oerviod tiaxuectionf a saddle \
shaped area of dorsal skin« wheace the scratch*reflex of the left hind limb can be evoked.
ir marks the position of the last rib.

B» Diagram of the spinal ara involved, l, receptive or afferent nervc^path from
the left foot; r, receptive nerve-path from the opposite foot; Ra, r^, receptive nerve* ]
paths from hair« In the dorsal skin of the left side; fc, the final common path^ in this \
case the motor neurone to a flexor muscle of the hip ; Fa, r^, proprio-sptoal neurones.

saddle-shaped field of skin (Fig, 13) excites a scratching move-
ment of the hind leg. The movement is rhythmic alternate
flexi-oo and extension at hip. knee, and ankle. Each flexion
recurs at a frequency of about four times per second. The
stimuli provocative '^ of it are mechanical, such as tickling the
skin or pulling lightly on a hair. The receptive nerve-endings

REFRACTORY PHASE

which generate the reflex lie in the surface layer of the skin,
about the roots of the hairs. A convenient way of exciting the
reBex is by feeble faradization, such as applied to one's own
tongue is felt as a tickling sensation. For exciting the reflex
electricaJly, I place a broad diffuse electrode on some indifferent
part of the surface outside the receptive field of skin, and apply

'7J

A B c

FiouRt 14. — Tracing of the flexion of the hip In the " scratdi-reflex ** of » ** spinal dog." la
A the redex is evok«d by tightly mblnng the skici at z point behind the shoulder, in B a '
C by unipolar faridization with we^ double-induction shocks applied to the same pointl
of skin through a needle point lightly inserted among the hair roots. Time marked ia
»Kond» betow. At the top in E and C an electric signal marks the double-induction
■bocks delivered, and at the bottom an electric signal marks the time of application of the

THE SIMPLE REFLEX

[Lect.

m. stigmalic electrode to some point in tiie saddle-shaped area of
dorsal skin. This electrode may consist of a minute needle, or
a gilt entomological pin ; it is inserted in the skin so lightly that
Its point just lies among the hair bulbs. If it be pushed further,
other types of reflex may be produced and the scratch-reflex be
inhibited. Prominent among the muscles active in this reflex
are the dorso-flexors of the ankle, the flexors of the knee, and
the flexors of the hip. If the rhythm of the last is recorded
graphically, tracings,*^ as in Figure 14, are obtained. It is then
demonstrable that the series of brief contractions succeed one
another at a rate the frequency of which is independent of that
of the stimulation. Thus, the rhythmic reflex is elicttable by the
application of a heat-beam or a constant current The make and
break of the current are especially able to excite it (Fig, 15), but
it is for a time maintained even by the continued passage of the
current, though it lapses in intensity until the current is broken.

II] REFRACTORY PHASE 49

when it appears temporarily with renewed vigor. At make and
break of the voltaic current the reflex-response is a short series
of rhythmic flexions.

The reflex is still more easily evoked by unipolar application
of high frequency currents ; it can be evoked with great vigour
and long duration by this mode of stimulation. Double-induc-
tion shocks applied at frequencies from once per second to Ave
hundred and twelve times per second and at various intermedi-
ate rates all evoke it easily ; so likewise do single break or make
shocks applied at rates varying in my experiments between 1.33
times per second and forty times per second.

Under all these various methods of excitation (heat-beam,
constant current, double and single induced currents, high
frequency currents, and mechanical stimuli) the rhythm of the
flexor response remains — so long as the internal conditions of
the reflex remain unaltered — almost the same.^^ It remains
so also when, instead of a regular succession, a grouped succes-
sion of stimuli is used for excitation, e. g.^ stimuli grouped in
twos and threes. It is obvious that this reflex exhibits a
refractory phase. Take the instance where the stimulus applied
consists of double-induction shocks, succeeding each other at a
frequency of one hundred per second. The reflex-arc in response
produces flesdon at hip about four times per second. So few
as three successive double-induction shocks will sufiice to excite
the reflex, but since in the instance taken about twenty such
shocks correspond in time with each flexor beat, and each such
beat is divisible into about equal periods of contraction and
relaxation, let us make the assumption — a liberal one — that
ten out of the twenty available shocks are serving as stimuli.
The arc, while the following ten are being applied, fails in spite
of them to excite the flexor muscles. To those ten it does not
respond at all.

Nor can this refractory state be overcome by simply increas-
ing the intensity of the stimuli. The reflex remains as perfectly
rhythmic and clonic under the strongest stimuli as under weaker
(Fig. 16). The frequency of the beats is under strong stimula-
tion often somewhat higher than under quite weak, especially at

so THE SIMPLE REFLEX [Lect.

outset of the reflex, but the difference is small, e. g.^ 5.8 beats per
second, instead of 4.5 beats per second. No mode or inten-
sity of stimulation to which I have had recourse converts rhythmic
clonic beat into a maintained steady contraction.* In this respect,
as in certain others, the scalptor reflex-arc closely resembles in
its behaviour the mechanism of the Medusa bell and heart-wall.
It resembles these more closely than it resembles the mamma-
lian respiratory reflex-arc, which can under several circumstances
be made to produce from the diaphragm an enduring tetanic
contraction.

I refer to the contraction of the flexor muscles in the scalptor-
reflex as a ** beat," not implying that it is a single twitch, but
rather because the term denotes a short-lasting phase of action
in a rhythmic series, and because of its close analogy to the
beat of Medusa and of the heart

What is the neuronic construction of the arc in the scalptor-
reflex? The reflex is in a sense unilateral ; stimulation of the
left shoulder evokes scratching by the left leg, not the right
Search in the spinal cord ^^ for the paths of the reflex demon-
strates that a lesion breaking through one lateral half of the
cord anywhere between shoulder and leg abolishes the ability
of the skin of that shoulder to excite the scratch-reflex, but
leaves intact the reflex of the opposite shoulder. In the lateral
half of the spinal cord which the reflex-path descends, severance
of the dorsal column does not obviously interfere with the
reflex ; nor does the severance of the ventral and the dorsal
columns together of that side ; no more does severance of the
gray, matter in addition. But severance of the lateral part of
the lateral column itself permanently abolishes the conduction
of the reflex ; and it does so even if all the other parts of the
transverse extent of the cord remain intact. The paths of the

Figure 16 (opposite). — Scntch-reflex, provoked by 42 break shocks deUvered at the nieof 40
per second. The intemiptions of primazy circuit producing these are recorded by electro-
magnet giving the top line of the record. As the stimulus is without other change rendered
more and more intense at 690, 11 00, 1900 and 3000 units of the Kronecker scale respec-
tively, the reflexes A B C D show the differences seen. Instead of one beat daring the
stimulus and two afterwards, as in A, the reflex gives three beats during the stimnlos and
six beats afterwards, as in D. Time in seconds below.

52 THE SIMPLE REFLEX [Lect.

reflex, therefore, descend the lateral part of the lateral column.
These details help towards construction of the reflex-arc in-
volved. For in the lateral part of the lateral column, as shown
by the method of successive degeneration^ lie long propruhspinal
fibres which directly connect the gray matter of the spinal seg-
ments of the shoulder with the spinal segments containing the
motor neurones for the flexor muscles of the hip, and knee, and
ankle. The course of the descending proprio-spinal fibres can be
traced and their number counted. The method of " successive
degeneration " **^ enables one to unravel them from descending
fibres of other sources, such as cerebral, mesencephalic, or bul-
bar, and fi'om proprio-spinal fibres descending from the foremost
segments of the neck. This is done — as the term " successive
degeneration" implies — in a preliminary lesion by severing
that part of the spinal cord it is desired to examine for the par-
ticular proprio-spinal fibres sought, from all the central nervous
system lying farther forward. To determine the proprio-spinal
fibres descending from the third and fourth thoracic segments,
the first step is to transect the cord between the second and
third thoracic segments. There then ensues throughout the
length of the cord behind that transection degeneration of all
the fibres that enter it from the brain, mid-brain, bulb, and cer-
vical and first two thoracic segments. This heavy degeneration,
after developing, reaches a maximum and gradually passes away,
all the debris of the degenerated nerve-fibres being in time
removed. For this a period of a year suffices in a dog. The
spinal cord is then ripe for determination of the proprio-spinal

Figure 17 (opposite). — Cross-sections of the spinal cord of the dog, revealing the position of
the nerve-tracts descending to the hind-limb region from origin in the foremost thxee thoradc
segments, by the method of " successive degeneration." The 8th cervical segment had been
exsected, and 568 days later a crosscut was made at the hindmost level of the 3d thoracic
segment The transverse extent of this lesion, as determined by microscopical sections
afterwards, is shown in diagram i of the figure. The greater part of the right lateral
column is seen to have been spared from injury. Three weeks subsequent to this second
lesion the animal was sacrificed. Preparations made with the Marchi method for reveal*
ing degenerate nerve-fibres showed the degeneration indicated by diagrams 2, 3, 4 and
5 in the figure. After the second injury to the cord the scratch-reflex remained elicitable
from the right shoulder, but was lost from the left shoulder in its anterior scapular region.
The degeneration of these proprio-spinal fibres descending from the shoulder segments
went, therefore, hand in hand with disappearance of the soatch-reflex from a region ol
skin of the shoulder whence it was elicitable previously.

SUCCESSIVE DEGENERATION

54 THE SIMPLE REFLEX [Lect.

fibres it is desired to examine. It becomes once more a clean
slate on which a new degeneration can be written. The proprio-
spinal fibres are revealed by making a transection between the
fourth and fifth thoracic seg^ments. Four weeks after this
second lesion the proprio-spinal fibres descending from the
third and fourth thoracic segments are degenerate. They can
be studied (Fig. 17) throughout their course from the fourth
thoracic segments backwards along the cord by any of the
ordinary methods, such as the Marchi method, for studying de-
generate fibres. Many of these proprio-spinal fibres pass from
the shoulder segments to end in the hind-limb segments. The
existence of the neuroglial scar, left by old degeneration that
has cleared up, far from complicating the tracing of the new
degeneration assists by forming a contrast background to it, the
sharpness of which leaves nothing to be desired.

From the results obtained by this method the following
reflex-chain can be inferred ^^ as possible and probable for the
scratch-reflex.

1. The receptive neurone (Fig. 13 B, Ra) from the skin to the
spinal gray matter of the corresponding spinal segment in the
shoulder.

2. The long descending proprio-spinal neurone (Fig. 13 B, Pa)
from shoulder segment to the gray matter of the leg segments.

3. The motor neurone (Fig. 13 B, F c) from the spinal seg-
ment of the leg to a flexor muscle.

This chain thus consists of three neurones. It enters the
gray matter twice ; that is, it has two neuronic junctions, two
synapses. It is a disynaptic arc.

In venturing to thus schematically express the construction
of this arc as disynaptic, I am influenced by the desire to ex-
press its construction as simply as possible so far as consistent
with the ascertained data of the case. I therefore omit from
the scheme the possible Schalt-Zellen (v. Monakow) between
Ra and Pa, and between Pa and F C. Much of what I intend
to express by " disynaptic " would be as clearly though not
as concisely expressed by saying that gray matter is inter-
calated in the arc twice i. e., at two separate places. That,

m REFRACTORY PHASE 55

however, is not expressible by a single adjective, and since
synapses occur so far as we know only in gray matter, disynap-
tic does include that idea. But it also implies somewhat more.
I venture upon it in spite of the assumptions it includes because,
in my opinion, much in those further assumptions seems justi-
fiable and useful as a working hypothesis, and because it lays
stress on the importance of the synapse in reflex conduction, —
an importance which for reasons given before seems considerable.

The reflex-arc consists, therefore, of at least three neurones.
It is convenient to have a term distinguishing the ultimate neu-
rone F C from the rest of the arc. For reasons to be given later
it may be spoken of as the final common pcUhJ^ The rest of
the arc leading up to the final common path is conveniently
termed the afferent arc.

The morphological components of this reflex mechanism
include, in addition to the above neural elements, the muscle-
fibres of the flexor muscle at one end, and possibly a receptive
cutaneous organ at the other end probably in the root-sheath of
a hair. Somewhere in this chain of structures the property of
refractory phase has its seat, and refractory phase is a pivot on
which the whole coordinating mechanism of this reflex turns.

In attempting to locate the seat of the refractory state, con-
siderations that arise are the following. The muscle involved is
one which neither when excited directly nor through its motor-
nerve exhibits this refractory period. We can exclude the
phenomenon, therefore, from it, and from its motor-nerve, and
from the link between it and its motor-nerve, — the end-plate.
Further, this refractory state is not exhibited when the motor
neurones of this muscle are excited to activity by various other
channeb; for instance, by the afferent neurones coming in from
the receptive organs of the leg itself, or by the palliospinal
P3rramidal neurones descending to them from the cortex of
the cerebral hemisphere. We are free, therefore, to exclude the
motor neurone F C supplying the flexor muscle itself as the
source of the refractory state characterizing the scalptor-reflex.
Again, the elicitation of the reflex typically (Fig. 14) by all the
various above-mentioned forms of artificial (electrical) stimuli

56 THE SIMPLE REFLEX [Lect.

applied through a needle electrode inserted in the skin sug^gests
that it is the commencement of the receptive nerve-fibres in the
skin rather than any specialized cutaneous sense-organ therein
that are the seat of stimulation when the reflex is thus artificially
excited. We have no knowledge of the existence of a reflectory
phase of such duration as this as a property of any afferent fibre
passing from the skin to the spinal cord. There is indeed con-
clusive evidence that the seat of this refractory phase lies neither
in the skin nor in the afferent neurone itself. When the reflex
is in progress under stimulation of the skin at one point, e. g.^ Ra
(Fig. 13, B), stimulation at some other, even remote, point also
producing the reflex — as can be proved in various ways — does
not break the rhythm of the reflex '^'t *» qj. complicate it in any
way. The reflex elicited from spot RiS may be initiated while that
elicited from Ra is in progress, and may then be carried on while A
is allowed to cease, or vice versa; but in neither case is the rhythm
of the reflex broken or reduplicated. And this although, fi-om other
evidence, we know that the flexor muscle and the motor neurone
F c can respond rhythmically to stimuli with a rate of rhythm
more than twice as high as that of the stimuli applied to produce
the reflex. Or the reflex from a spot B may be introduced into
the middle of one from a spot A, and although its reflex (B)
impresses characters of its own as to amplitude, direction of the
foot, etc., it does not reduplicate or break up the already exist-
ing rhythm. The result does not differ whether the individual
stimuli of the two series of stimulations at spots A and B are
alternate or not. Figures 18, 19, 20, 21 illustrate these points.
The refractory phases obtaining in the reflex arising from spot
A are respected by the stimuli delivered at B also and are not
broken down by them.

There is evidently some part of the reflex mechanism which

Figure 18 (opposite). — Tracing of the flexion of the hip in the " soatdi-reflex." The reflex is
evoked by two separate stimulations (unipolar faradization) at points ten centimeters apart
on the skin surface. The upper signal shows the time of application of the first stimulation,
and the line immediately below that the frequency of repetition of the double-induction
shocks of that stimulation. The lowest line signals the time of application of the second
stimulation : the frequency of repetition of the double shocks in this stimulation was mudi
greater than in the other stimulation and is not shown. At the top the time is marked in

PfOtrms Jf . — Rocnrds of scratch-r«flex as before, evoked by separate unipolar stimtiktSoB of
two skin points 8 centimeters apart. In A tht second stimulation commences during the
after-discharge of the first reflex and causes no interruption or alteration in the rhythm of
the reflex-response. In B the second stimulation of the above pair evokes the reflex^ and
during its progress the stimulation which comes first in A is introduced ; and converftely
in C. Id none of the three cases is there interruption of or alteration in rate of the rhythmic
reflex-response. Rate of delivery of the excitiiig shocks is shown on separate lines,

against what we know of the receptive neurones to suppose that
there is between theni any collateral nexus beyond their impinge-
ment directly or indirectly on £>/A/r neurones more or less common
to them both. The seat of the refractory phase seems therefore
to lie somewhere central to the receptive neurones in the affer-
ent arcs. The refractory phase induced in some element of the
arc by the reflex from A extends to some element which is also
concerned in the conduction of the reflex induced from B. This
element must be some neurone common to the two arcs from A
and B respectively. Neurone FC (Fig. 13), the final common path,
is such an element But neurone FC as tested by other reflexes,
€, g,, the flexion-reflex shows no such refractory period. The
common mechanism sought for seems therefore to lie some-
where between FC and Ra. R/9. It may well be that neurone Pa

II] VALUE OF REFRACTORY PHASE 6i

is partly common to Ra and R/3, for these R neurones are well
known to split intraspinally into headward and tailward stem-
fibres, each carrying many collaterals, and probably by them con-
nected with the gray matter in not only one spinal segment but
in a series of segments. Collaterals from R/8 as well as from
Ra may reach Pa, therefore ; and similarly with P/8.

The scratch-reflex has instructive points of likeness to that
of the swimming-beat of Medusa. The arrangement of its
response is quite like that of the muscular response of the
swimming-bell of Medusa under stimulation of two points of
the subumbrella or two of the marginal receptor organs. We
can compare each lateral half of the saddle-shaped receptive
area of the dog's back in regard to the scratch-reflex quite
strictly to the marginal surface of Rhizostoma in regard to its
swimming-beat reflex.

In Medusa a second stimulus following close upon a first
does not prolong the contraction ; ^ it finds the bell in refractory
phase. The beat induced by the first stimulus has no second
contraction fiised with it in consequence of the second stimulus.
The principles of the co-ordination thus obtained in the rela-
tively simple swimming of Medusa seem as follows.

One condition of the co-ordination of the swimming-beat,
as said above, is the unpolarized nature of the nerve-channels
allowing free flow of nervous impulses in either direction from
conductor to conductor along the nerve-net Another condition
is the continuity mediate or immediate of every conductor with
every other. These conditions of the nervous system allow a
single stimulus given at any single point to evoke a co-ordinate
contraction of the whole musculature, to evoke, in short, a full
and perfect swimming stroke or beat. But they do not insure
that under a series of stimuli delivered in irregular and varied
sequence and at various points in perhaps rapid succession a
series of co-ordinate strokes or beats shall result.

Romanes ^ showed that the receptor organs at the edge of
the bell were the source of the natural beats of the bell, and that
so long as even one of these remained the swimming-bell con-
tinued to beat spontaneously. There therefore exist in this case

62 THE SIMPLE REFLEX [Lect.

quite a number of points which all tend under natural circum-
stances to initiate the beats of the bell.

Suppose that shortly after a stimulus has occurred at A, and
before the contraction induced by it has passed off, another
stimulus is delivered at B, and then similarly at C. All these
stimuli are, taken singly, similarly provocative of locomotion,
and the only locomotive act of the creature is the systolic stroke
of its bell. Each reaction is therefore aimed at incitement of
locomotion only for a propulsive movement of the bell. But
the two conditions, i. e.^ unpolarized conduction and end-to-
end continuity obtaining in the nerve-net, though they insure
that result for one stimulus, defeat it in the case of a series of
stimuli quickly following either at one and the same point or at
several, — and for the following reason. The series of stimuli
would, were those two conditions all, merely immobilize the
bell. The second stimulus following during the contraction
excited by the first would be conducted, as was the former, to
all the musculature of the movement, and would simply accentu-
ate and prolong the systolic condition already in progress. But
that would interrupt locomotion, not promote it. The con-
traction of the muscular disc to produce the stroke must be
immediately preceded by diastole, enabling it to embrace the
proper volume of water for re-expulsion.

An essential feature of the co-ordination is therefore due to
alternation of the two converse phases of contraction and relax-
ation in the one muscle ; these execute the locomotion of this
simple animal. This necessary condition is insured, even under
irregular series of stimuli, by refractory phase.

When the bell is replying or has just replied to a stimulus, it
remains inexcitable to further stimuli for a period which outlasts
its phase of contraction. This prevents disharmony occurring
in the rhythmic movement under multiple stimulation.

It is on conditions like these governing Medusa's swimming-
bell that the co-ordination of the heart's action is based, except
that in the heart there is but one initiatory spot, prepotent per-
manentiy, as in Vertebrates, or temporarily, as in Tunicates. In
Medusa presumably any one of various specialized points (border

11] PERIPHERAL. CENTRAL CO-ORDINATION 63

organs) becomes prepotent at different times, by reason of stimu-
lation from the environment. In this Medusia more closely re-
sembles the scratch-reflex, where any one of many points in a
large receptive field becomes temporarily prepotent under stim-
ulation, €. ^., puncture by a flea or other parasite, and then
initiates and leads a series of beats which can be prolonged
or intensified by concurrent stimulation by parasites at other
points, but cannot by their concurrence be upset as regards
rhythm. In the action of scratching it is as necessary as in the
swimming of Medusa, or the beating of the heart, that relaxa-
tion follow contraction.

Refractory phase is obviously an essential condition in the
co-ordination of the scalptor-reflex. The scratching-reflex, in
order to secure its aim, must evidently consist of a succession
of movements repeated in the same direction, and intervening
between the several members of that series there must be a
complemental series of movements in the opposite direction.
Whether these two series involve reflex contractions of two
antagonistic muscle-groups respectively in alternate time, I would
leave for the present The muscle-groups or their reflex-arcs
must show phases of refractory state during which stimuli can-
not excite, alternating with phases in which such stimuli easily
excite. Evidently this is fundamental for securing return to the
initial position whence the next stroke shall start. The refrac-
tory phase secures this. By its extension through the whole
series of arcs it prevents that confusion which would result were
refractory phase in some of the arcs allowed to concur with ex-
citatory phase in others.

But there is one significant difference between refractory
state in the scratch-reflex and in the swimming mechanism of
Medusa. In the latter, as in the heart, the refractory state is a
property not relegated to a central nervous organ remote from
the peripheral tissue in whose function it finds expression. It
is located in intimate connection with the peripheral organ itself.
From observations of Bethe it seems likely that refractory phase
in Medusa is a function of the nerve-net. Magnus ^'^ has re-
cently shown that the refractory phase of the beat of the isolated

64 THE SIMPLE REFLEX [Lect.

intestine is referable to the local nerve-plexus (Auerbach's) lying
in the gut wall. In these cases the refractory state seems to
belong to the nervous elements, but to nervous elements diffused
through the peripheral tissue. But in the scratch-reflex the
site of the refractory state is central, intraspinal.

The centrality of seat of the refractory state of the scratch-
reflex is significant of the difference of the conditions under which
the scratch-reflex and the swimming of Medusa respectively go
forward. In the case of the locomotor action of the swimming-
bell of Medusa, we have a simple musculature which can execute
practically but one movement It is in fact a single muscle, that
is to say, comparable with what in the more complex muscula-
ture of higher organisms — e. g,, vertebrates — is regarded as a
unit of musculature, a single muscle, such as the gastrocnemius^
tibialis, etc., in the frog. Each and every receptor organ which
under stimulation produces locomotion is therefore connected by
nerve with that single muscle of locomotion, and when impelled
by each or any of them, the muscle effects practically the same
action as it does when impelled by any other of the sister receptor
organs. The movement of locomotion which is provoked through
each receptor is practically the same as that provoked through
any of the rest. The mechanical organ in this case can perform
but one movement, and its performance of that movement is,
so to say, the one purpose demanded from it by each of all the
receptor channels playing upon it.

But with the mechanical organ which the scratch-reflex
employs the case is different. That organ is the hind limb, a
complex structure built of parts, many of them spatially op-
posed, and able as a whole to execute movements of various
kinds. Thus it can reflexly not only scratch, but stand, walk,
run, or gallop, squat in defaecation, abduct and flex in mic-
turition, etc. In the swimming-bell of Medusa there is no
opportunity for antagonism between the motor end-results of
the reflexes that employ it, save in respect to the possible
confusion of successive contractions which would destroy the
rhythmic pulse, and that confusion is avoided by refractory
phase. The swimming-bell of Medusa is at the behest of but

II] SEAT OF REFRACTORY PHASE 65

one type-reflex. The scratch-reflex possesses the same safe-
guard against destruction of its rhythmic character. But in the
case of the scratch-reflex that reflex is but one of several re-
flexes that share in a condominium over the effector organ —
the limb. It must therefore be possible for the scratch-reflex,
taken as a whole, to be, as occasion demands, replaced in exer-
cise of its use of the limb by other reflexes, and many of these
do not require clonic action from the limb, — indeed, would
be defeated by clonic action. It would not do, then, for the
j:>eripheral orgaji itself to be a clonic mechanism. The clonic
xnechanism must lie at some place where other kinds of reflex
^ran preclude the clonic actuator from affecting the peripheral
^^rgan. Now such a place is obviously the central organ itself;
Wot that organ is, as its name implies, a nodal point of meeting
"9o which converge all the nervous arcs of the body, and among
others all those which for their several ends have to employ the
same mechanical organ as does the scratch-reflex itself. It is
therefore only in accord with expectation that the seat of the
refractory phase of the scratch-reflex lies where we traced it, in
the central nervous organ itself, and somewhere between the
motor neurone to the muscle and the receptive neurone from the
skin. For it is upon the motor neurone that other arcs impinge.
The reflectory state is obviously akin to a state of inhibition,
and just as there are well-known examples in which the inhibi-
tory state is peripheral (^.^., the heart), and others in which the
inhibition is central, so undoubtedly phases of refractory state
are in some instances peripheral, but also in numerous instances
are central, — and this is so in certain reflex actions.

The reflexes of which refractory phase constitutes a promi-
nent feature are those concerned with cyclic actions occurring
in rhythmic series ; such as the scratch-reflex, reflexes of swal-
lowing and blinking, and probably the rhythmically recurring
reflexes concerned in the stepping of the limbs.

Nothnagel (1870),^ following up an observation by Sets-
chenow,*^ studied a periodic and rhythmic reflex of the crossed
hind limb in the spinal frog. He found that if some days after
the frog's cord had been transected at the fourth vertebra, the

THE SIMPLE REFLEX

central end of the sciatic is faradized, a rhythmic alternating
flexion and extension of the opposite hind limb is evoked. He
noted that no intensity of stimulation makes the rhythmic move*
ment alter from clonic to tonic. A reflex of this kind is, I
findi elicitable in the spinal dog^s hind leg by unipolar faradiza-
tion of the opposite foot, and, as Nothnagel noted in the frog»
no mere increase of intensity of stimulation converts its clonic
character into maintained tonic. It is rhythmic (Fig, 22), and
has a refractory period which no ordinary increase of inten-
sity of stimulation suffices to break down. The frequency of its
rhythm averages 2.3 per second, but varies somewhat in different
observations. This is about twice as slow as the frequency of
the scratch- reflex, which averages 4.5 per second. The average
rhythm of the scratch-reflex is almost exactly the same as that
ascertained by Gotch and Burch ^** for reflex-discharge from
the electric cell of Malapterurus, but the rate in Malapterurus
appears to vary much more than that of the scratch-reflex.

THE STEPPING-REFLEX

FiGtrvx 33. — Th« ** extensor-thrust " ; spitu^ dog. Time betow in tecoMb.

There is a peculiar brief extension-reflex of the dog's hind
leg which I term^^ the ** extensor-thrust." Baglioni^^ has
more recently noted an analogous reflex in the frog. This
reflex is elicited by mechanical stimuli applied to the ptanta.
In the spinal dog, where well marked, it is often elicitable by
even lightly stroking with the edge of a piece of paper the skin
behind the plantar cushion. It is more certainly e%'oked by
pushing the finger-tip between the plantar cushion and the toe-
pads, especially when the hip and knee, not necessarily the
ankle, are resting, passively flexed.

The " extensor- thrust " I have never succeeded in prolonging
to a full half-second — none of my records exhibit it as long as
that Usually the duration as recorded is about a fifth of a second
(Fig. 33.)* Possibly its muscular contraction may be a simple
twitch, though reflexly excited. Its muscular field involves the
muscles of the ** knee-jerk." The myogram of the extensor
thrust is shorter than that of tetanic contraction of the dia-
phragm (Head's slip) "• caused by two successive stimuli to the

THE SIMPLE REFLEX

[Lect.

phrenic nerve when the muscular responses of the two just fuse
(Fig. 24)-

Immediately after its clicitation this reflex, in my experience*
remains in the spinal dog for nearly a second relatively inelicit-

FiGun* 34, — Afyogram of the contraction of the diaphragm of the rabbit (Head's slip)
dicit^d by two break shocks apphed to the phrenic nerve. The momenta ol tpplication of
the two shocks zrt indicated by abscissae on the myograph curve. Time below in han>
dredths of a second ( Macdooald and Sherringtoa).

able. Its reflex-arc exhibits after its phase of activity a refrac-
tory phase. The refractory phase is here far longer than that
of the scalptor-reflex. It may last six times as long as the
period of activity; thus the extensor-thrust may last only I70<r
while the succeeding refractory phase may endure a full second.
The extensor-thrust is probably an important element in the
reflex mechanism of the dog's locomotion.* One peculiarity it
has as compared with other spinal reflexes of the limb is the
considerable force which it exerts. In the locomotion of the
animal it provides much of the propulsive power required,
t Compare on this the recent paper by M. Philippsoii.*°^

II] THE EXTENSOR-THRUST 69

Bearing tbese points in mind, it is obvious that as an element in
locomotion its repetition is required only at intervals consider-
ably longer than the duration of the thrust itself, namely, at a
particular phase of each successive step taken in the progression
of the animal After the extensor-thrust, the limb has to be
given over to the flexor muscles in order, without touching the
ground, to swing forward in preparation for the next step by the
limb. It is reasonable to suppose that part of the means by
which selective adaptation has secured this result is the evolu-
tion of the long refractory phase following the activity in the
reflex-arc of the extensor-thrust Zwaardemaker*^ has shown
that the reflex movement of swallowing in the narcotized cat is
followed by a refractory period,^ lasting half a second or longer.
This refractory state is central, for when the reflex swallow has
been elicited by the superior laryngeal nerve of one side, the
after-lasting refractory period holds good also for excitation of
the opposite superior laryngeal nerve.

Variation of the external stimulus has comparatively little
effect upon the length of the refractory period. But internal
conditions, such as blood supply, fatigue, narcosis, etc., do
influence it greatly. For reflexes which exhibit a refractory
phase, a certain duration of that phase, subject to some varia-
tions, is characteristic. The duration of the phase varies con-
siderably in different types of these reflexes.

It is clear that an essential part of many reflexes is a more or
less prolonged refractory phase succeeding nervous discharge.

Refractory phase appears therefore at the one end and at the
other of the animal scale as a factor of fundamental importance
in the co-ordination of certain motile actions. In the lowly
animal form (Medusa) it attaches locally to the neuro-muscular
organ, and so also in the visceral and blood-vascular tubes
of Vertebrates. But in higher forms (dog) refractory phase
occurs as regards the taxis of the skeletal musculature, not in
the peripheral neuro-muscular organ, but in the centres of the
nervous system itselfl

' BagUooi has pointed oat refractory phase in a reflex in the frog.**

70 THE SIMPLE REFLEX [Lect.

LECTURE III

CO-ORDINATION IN THE SIMPLE REFLEX {cofuiuded)

Argument: Correspondence between intensity of stimulus and intensity
of reflex reaction. Differences between different reflexes in this re-
spect Functional solidarity of the intraspinal group of elements
composing a reflex " centre." Sensitivity of reflexes, as compared
with nerve-trunksy to asphyxial and anaemic conditions, and to an-
aesthetic and certain other drugs. Functional significance of the
neural perikarya. Reflexes of double-sign. Reflexes of successive
double-sign, and of simultaneous double-sign. Evidence of recipro-
cal innervation in reflexes. Reflex inhibition of the tonus of skeletal
muscles. Reflex inhibition of the knee-jerk. Time-relations and
other characters of reflex inhibition as exemplified by the flexion-
reflex. Other examples of inhibition as part of reflex reciprocal
innervation. The seat of this reflex inhibition is intraspinal Conver-
sion of reflex inhibition into reflex excitation by strychnine and by
tetanus toxin. Significance of the '^ central " situation of reflex in-
hibition in the cases here dealt with.

Ohrading of intensity. A further diflference between the re-
action of a reflex-arc and that of a nerve-trunk lies in the
greater ease with which in the latter the intensity of eflfect can
be graded by grading the intensity of the stimulus. In the
nerve-trunk this has been examined both with the action-current
(Waller) ^^ and for motor-nerves by the muscular contraction
(Pick, Cybulski, and Zanietowski).^** The accuracy of grading
within a certain range of stimulus-intensity is so remarkable
that the ratio between stimulus-intensity and response-intensity
has by some observers been assigned mathematical expression.
Waller finds the response in a nerve-trunk, directly stimulated,
increase in much closer direct proportion to the increment of
external stimulus than does the response of muscle to indirect
stimulation, or the response of the optic nerve when the retina
is adequately stimulated. The correspondence between inten-
sity of external stimulus and reflex end-effect is again less

mj GRADING OF INTENSITY OF REFLEX 71

close still; indeed it is often stated that reflex reactions re-
semble as to intensity the ''all-or-nothing" principle of the
cardiac beat (Wundt). Biedermann remarks of reflexes evoked
by single-induction shocks in the cooled frog, that there is
practically no grading of intensity: they are all maximal. Bag-
lioni makes the same remark for other reflexes in the frog.

Yet graded intensity of reflex-efiect does occur. Walton •*
noted as one of the features of strychnine poisoning, that at
a certain stage the grading of intensity is lost and all reflexes
become maximal. Merzbacher^^ and Pari'^ have supplied
some measurements of the increment in amplitude of the reflex
movements of the frog's leg under increase of intensity of
stimulation.

The flexion-reflex of the hind limb of the spinal dog in-
creases in amplitude in correspondence with increase of intensity
of stimulus — alterations of time-relations of stimulus being ex-
cluded. Increase of intensity of stimulus heightens the reflex --
contraction both in power and amplitude. Figure 25 shows a
successive series of these reflexes, each elicited by a series of
break shocks delivered at the same skin-spot by a stigmatic
kathode. The increments of the reflex run fairly steadily with
the tip-gradient of intensity of stimulus.

In the scratch-reflex a grading of the intensity of the re-
flex is easily obtainable by grading the intensity of the stimu-
lus. By a suitably weak stimulus a scratch-reflex can be
elicited which exhibits but a single beat. Increase of intensity
of the reaction does not show itself in increase in frequency of
the rhythm of this reflex, or shows itself very slightly in that
form, the refractory period being hardly curtailed at all. The
increase reveals itself as greater amplitude of the individual beats
of the rhythmic contraction. By simply bringing the secondary
coil nearer to the primary in a dozen successive steps, it is easy
to obtain a dozen grades of amplitude in a dozen successive
examples of this reflex (Fig. 26). The beats in response to a
strong stimulus may have six times the amplitude of those evoked
by a weak. The single beat that can be obtained by a suitably
feeble stimulus (Fig. 9) is not only small but slow ; it resembles

^^ THE SIMPLE REFLEX [Lect.

tbe last beat the reflex gives as it dies out after cessation of an
ordinary stimulus. The feeble, slow character of the terminal
beat of the ordinary reflex is not therefore due to fatigue, but
simply to weak intensity of excitatory process at the moment.

The scratch-reflex, though it resembles the heart beat in
relative immutability of rhythm under change of intensity of
stimulation, differs from it in the change of intensity of its beat,
which follows change in intensity of stimulus. It does not ob*
serve the " all-or-nothing " principle. It is obvious that in the
heart beat the object is to put a pressure on the contents of
the ventricle higher than that obtaining in the aorta, and that
aim reached, any further excess of pressure is useless or harmful,
for it subjects the heart and the arterial wall to an unnecessary
strain. Clifibrd Allbutt^^ remarks: "It is the function of a
healthy heart and arteries to promote the maximum of blood
displacement with the minimal alteration of pressures." The
stress under which the heart is driven is less closely associated
with intensities of stimulus than with conditions internal to itself,
€. g., distension, etc. But with the scratching movement it is
obvious that a strong scratching movement may remove an irrita-
tion more quickly and more effectually than weak movement

On the crossed extension-reflex the effect of increase of in-
tensity of stimulus shows, in my experience, somewhat differently.
After a relatively slow and gradual increase of reflex-response,
there appears at a certain intensity of stimulation a sudden rela-
tively large increase of the response. This augmentation is

Figure 25. — The flexion-rcflez, showing gndient of intensity due to pided intensities of
stimulus. The time of tlie stimulus is marked by the signal line above ; the stimulus con-
sisted in each case of la break shocks delirered at the rate of 25 per teoond, applied by uni-
polar method with kathode needle point in skin of a digit, the diffuse pole lying headward
of the spinal transection. The intenral between commencement of each reflex was two
minutes.

InUntify ^stimmhu. H tmtmmr ^r^fUx

A. 690 8.5

B. 3000 59

C. 5200 no
D 9800 168
E. 12500 213
B|. 3000 29

Intensity is given in units of the Kronecker scale. Time marked below in seconds
Prom same animal as jrielded observations of Kg. 44 but on the succeeding day.

74 THE SIMPLE REFLEX [Lect.

chiefly in the form of " after-discharge " (Fig. 27). This reflex
shows well that internal conditions play a greater rdle as com-
pared with external in reflex conductions than in nerve-trunk
conduction. Under strong stimuli the augmentation of the
reaction by after-discharge becomes enormous.

The " extensor-thrust " I have failed to evoke by any stimulus
easy to grade or record a measure ofl But my experience of it
under the particular form of mechanical stimulation it appears
to require leads me to think that strength of external stimulus
aflects the reflex-response but littie, and that this reflex does
much resemble the heart in responding either not at all or fully.
Its graphic record time after time in a series of stimulations re-
peats itself with very little difference of character.

Therefore, from the reflexes of the limb of the spinal dog, it
would appear that in respect to ability to be graded in intensity
in accordance with grading of intensity of stimulus, there exist
great differences between the various type-reflexes. Some re-
flexes — e,g.y the flexion-reflex and the scratch-reflex — easily
exhibit grading, while others do not The difference between
reflex-conduction in various reflexes in this respect may explain
the discrepancies between various observers on this point

A factor in the grading of submaximal eflfects in muscle and
nerve by weak stimuli may well be limitation to certain of the
component fibres, whereas a maximal stimulus excites them all.
It is a matter of interest how far this numerical factor explains
submaximal responses from a spinal centre. If its elements are
functionally separate, partial responses are open to occur in such
a mechanism since it is multiple as regards its physiological
components. Gotch^^ has recently raised this question in an
interesting way. He points out that in the electrical organ of
Malapterurus, where the whole organ is innervated by a single
nerve-fibre^ the reflex-response is little variable in its intensity
in comparison with the wide range of response to increasing
stimulus-intensities exhibited by stimulation of the electric nerve
of Torpedo, — a structure containing many nerve-fibres. Con-
cerning the grading of motor discharges from the central nervous
system he asks, — " Is it not possible that these grades are

76 THE SIMPLE REFLEX [Lect.

largely dependent on the number of central elements involved,
and are only incidentally associated with variations in the in-
tensity of the nervous process in any one neurone?"

That in spinal reflexes increase of the intensity of the excit-
ing stimulus causes increase in the number of motor neurones
excited is clearly shown by the wider field of musculature seen
to be engaged as the reflex irradiates under intenser stimulatioii.
This is the well-known spread which Pfliiger** endeavoured to
formulate rules to express. Within one and the same musde-
groups, and even within one and the same individual muscle,
grading of intensity of reflex contraction by this numerical im-
plication of more or fewer motor-cells seems not only possible
but probable. It is perhaps one object of their multiplicity.
The want of difference between the latent time of the incre-
mental and initial reflexes mentioned above (Lect !» p. 24)
might be explicable thus.

With very feeble stimuli, or under spinal shock, when only
feeble reflex reactions can be evoked, it is easy to see /ar^lMi/ con-
tractions of muscles, e. g., in the tibialis anticus, sartorius» and
semimembranosus. Under like circumstances the scratch-reflex
may have the form of simply a feeble rhythmic dorsi-flexion at
ankle and toes, or even of the toes alone. We have referred to
such phenomena before, and they harmonize well with the view
of a fractional activity of the motor centre of the scratch and
other reflexes.

Yet we must not lose sight of the physiological solidarity of
the action of the group of elements that compose a " reflex
centre " in its reflex activity. '' Immediate spinal induction "
{v. infra, Lect. IV) and the spatial spread of the refractory
phase in scratch-reflexes evoked from separate points show
(Lect. II, p. 60) that, intraspinally, the various component arcs
of the type-reflex are interconnected to something like a unitary
mechanism. Further, the evidence that when the scratch-reflex
is being elicited from one point the refractory period obtains

Figure 37. — The crossed extemioii-reflex showing grades of intensity corresponding with
grading of intensity of stimulus. The time of stimulation is shown by the signal oa tht
line abore, and in each case consisted of 12 break shocks delircrad at the rate of 25 per

OF INTENSITY OF REFLEX

78 THE SIMPLE REFLEX [Lect;

practically throughout its intraspinal centre indicates the same
functional unity. The nerve-cells building the centre seem
combined like those of the nerve-net of Medusa. The ele-
ments seem incapable of isolated excitation. Such an intra-
spinal group as that involved in the scratch-reflex must extend
through a considerable length of the cord. And yet though
interconnected, like the web of Medusa's nerve-net, there is
evidence that in the various reflex forms which the scratching
movement takes according as elicited from one point or an-
other, one part of the centre is the more active in one form of
the reflex and another in another form of the reflex. The
mechanism is not therefore always equally aflected through-
out, and in this inequality numerical proportion of active to
inactive elements may play a part The mechanism, neverthe-
less, although anatomically an assemblage of units, is function-
ally itself a unit And a comparable solidarity obtains in other
reflex mechanisms. Even the irradiation which suggests exten-
sion to new units itself gives evidence of the welding of the
unit elements of centres together into functional unit groups
possessing solidarity. When in the flexion-reflex the response
spreads from the knee to the hip, the spread is not gradual,
but the hip flexion suddenly comes in, marking a sharp step-
like rise on the record (Fig. 45, p. 153). It is not as though the
irradiation gradually reached the motor elements of the hip-
flexion centre cell by cell: the irradiation on involving that
centre forthwith evokes discharge from it which, judging from
its powerful effect, represents discharge from the centre practi-
cally as a whole. The reaction as it irradiates treats the centre
as a unit.

The great prolongation of the reflex-discharge produced by
intensifying, apart from prolonging, the external stimulus is
also against the increase of discharge being explicable merely
or chiefly by implication of a greater number of motor ele-
ments. In the flexion-reflex, the period of discharge may be
lengthened tenfold by increasing simply the intensity of the
stimulus without lengthening it. In the " crossed-extension
reflex " I have seen the period of discharge lengthened more

Ill] OXYGEN AND REFLEX CONDUCTION yg

than twenty-fold. This argues that the grading of the motor dis-
charge in these reflexes is in important measure due to graded
intensities of discharge from the unit elements themselves, of
which the reflex centres are compounded. The functional unity
of a reflex centre seems also evident from the fact that it is
the instrument of a number of receptive organs scattered over
a relatively wide field — a field which for the flexion-reflex is
almost coextensive with the whole skin surface of the limb —
and nevertheless an intense stimulus from any limited part of
that field can elicit a reflex of full strength. This it can only
attain if the whole centre of the reflex be at its disposal. There-
fore, as we saw in the scratch-reflex, the whole motor centre
potentially belongs to all and each of the groups of receptive
organs proper to the reflex. The centre, although consisting of
anatomical units which are individual, seems knitted together
functionally. It is not necessarily the motor cells which conjoin
— were that so one hardly sees how stimulation of the central
end of a motor root could fail to excite discharge from other
motor roots, which the Bell-Magendie law shows that it does
actually fail to do.

Raflez oondnction less resistant than nerve oondnotioQ. The
difierences traced thus far between reflex-arc conduction and
nerve-trunk conduction have been differences brought out by
variations of stimulus and other external conditions. Differ-
ences no less notable appear under changes of internal kind.
Without entering on these fully, a glance at them is helpful
for the understanding of reflex-arc conduction.

Conduction by nerve-trunks is but slowly affected by inter-
ference with blood-supply ; reflexes are nevertheless among the
earliest reactions to alter or fail under asphyxial conditions.
V. Baeyer** found the sciatic nerve of the frog retain excitabil-
ity and conductivity three to five hours in nitrogen, and on read-
mission of oxygen regain its powers in a few minutes. Bergmann
(cited by Biedermann) found interruption of the circulation in the
frog extinguish reflexes in thirty minutes. Verworn has shown
that the spinal centres of a strychnized frog, if unsupplied with
oxygen, fail to react in about an hour's time, but are promptly

8o THE SIMPLE REFLEX [Lect.

restored by resupply of oxygen. Baglioni ** has shown that
the spinal centres of the frog, deprived of circulation and im-
mersed in nitrogen, fail to give reflexes in about forty-five
minutes, but in an atmosphere of oxygen continue to react
for twenty hours.

Again, the dosage of chloroform or ether required to depress
and abolish nerve-trunk conduction is much greater than is
required to depress and abolish the cerebro-spinal reflexes. In
Waller's observations on extinction of action-current in nerve-
trunks a 3 per cent dose of chloroform in air was required.
Using in the cat indirect contraction of the gastrocnemius as
an index of sciatic nerve-conduction, Miss Sowton and m>'self
found .3 per cent chloroform in diluted blood at 36** C. re-
quired to abolish the reaction (Fig. 28). This is a much
higher percentage than suffices to depress the heart's action.
Since many reflexes are abolished by doses which do not
markedly depress the heart, reflex conduction is abolished by
doses a fortiori smaller than those which set aside nerve-trunk
conduction.

Again, a number of agents, e, g.^ strychnine, tetanus toxin,
etc., that do not appreciably afiect nerve-trunk conduction
enormously alter reflex-arc conduction. All these seem to
exert their influence on some part of the reflex conductor which
lies in gray matter. It is interesting to ask whether they, e, g.

Figure 28 (opposite). — A. Hind limb of cat. Perfused with dilate blood. Contractions of gas-
trocnemius muscle stimulated through its nerve. Effect of CHClg at 0.35 per cent. The
register of flow of the blood through the blood-vessels shows a diminution at first, and then
a marked increase. The register of flow is the bottom line : each notch in that line in-
dicates one emptying of the Schafer ** tilter '* receiving the Uood at outflow from the limb.
The line next above marks the time in intervals of fifteen seconds. The third line from
bottom signals the perfusion of blood containing chloroform 0.25 per cent; similar blood,
but free from chloroform, being perfused before and after. The top line indicates the
pressure of delivery of the perfused blood at the entrant cannula. The chloroform reduces
the contractions of the muscle, stimulated through its nerve, by more than a half.

B. Same as above. Contractions of gastrocnemius muscle stimulated alternately, through
its nerve, and directly. The nerve was inexcitable before the perfusing fluid was turned on,
and the record which begins immediately after perfusion had been started shows the giadual
recovery of excitability. CHCl s at 0.3 per cent abolishes the response of the musde to
indirect stimulation, and reduces its direct response almost to zero. The second, fourth,
sixth, etc., are direct responses of the muscle; the first, third, fifth, etc, are responses to
stimulation through the nerve-trunk. In this tracing the time record is above the signal
record. (Sowton and Sherrington.)

mj CHLOROFORM AND REFLEX CONDUCTION 8i

82 THE SIMPLE REFLEX [Lect.

strychnine, have an effect similar to their spinal effect when
exhibited in Bethe's preparation of the second antenna ganglion
of Carcinus, whence the motor perikarya have been removed.
If these agents have their locus of incidence at the synapse, it
must be conceded that they act with very different intensities
at different synapses.

From this rehearsal of the differences between nerve-trunk
conduction and reflex-arc conduction it seems evident that cer-
tain elements of co-ordination of ** the simple reflex " are to be
found in the qualities of conduction of the reflex-arc Each of
the various types of simple reflex possesses to a large extent its
own peculiarities of conduction. Though there are differences
between conduction in various nerve-trunks, e. g.^ in speed of
transmission of impulses, etc., these differences sink to insignifi-
cance when contrasted with the extent and variety of the con-
ductive differences exhibited by different reflex-arcs. And in the
case of each reflex-arc its idiosyncrasies of conduction form an
obvious basis for the co-ordination exhibited by its reflex-act

Fnnotions of the perikaryon. — It may appear that our tend-
ency is to attribute the distinctive characters of reflex-arc
conduction so liberally to the synapse that the perikaryon
is stripped of all functions and only equivalent to a piece of
nerve-fibre. But it is to be remembered that two functions
of great importance certainly belong to the perikaryon. In
the first place it, even if the conductive process in it be wholly
similar to that of a nerve-fibre, is at least a place where the con-
ductor branches, often to such an extent as occurs nowhere else,
so that it is a nodal point in the spatial distribution of the con-
ductive lines. In the second place, there seems no valid reason
yet to doubt the long-held view that regards the perikaryon as
the nutritive centre of the neurone to which it belongs.

Certain features mentioned already as saliently distinguish-
ing reflex-conduction from nerve-trunk conduction still remain
for consideration. Among these are fatigability, facilitation,
inhibitory interference, spinal induction. These will, however,
be better taken under the compounding of reflexes. One feature

ni] RECIPROCAL INHIBITION 83

that we have not considered may, however, with advantage be
considered at once. This feature is inhibition.

Redprooal inhibition. — In the end-effect of certain reflexes,
for instance the scratch-reflex, there supervenes on a phase of ex-
citatory state a state refractory to excitation — a refractory phase.
This refractory phase is, if we seek to put it into the class of
physiological phenomena to which it must obviously belong, a
state of inhibition. In the scratch-reflex we have therefore a
reflex in which an external stimulus evokes as its end-effect an
excitatory phase, succeeded by an inhibitory phase, and this suc-
cession in this reflex, the stimuli being continued, is repeated
many times. If we denote excitation as an end-effect by the
sign plus (+), and inhibition as end-effect by the sign minus (— ),
such a reflex as the scratch-reflex can be termed a reflex of
double-sign, for it develops excitatory end-effect and then in-
hibitory end-effect even during the duration of the exciting
stimulus.

There is a further numerous class of reflexes in which
the end-effect consists both in excitatory state and in in-
hibitory state, but the inhibitory state does not supervene
on the excitatory or have the same locus of incidence as the
excitatory; it occurs simultaneously with it at another inter-
related locus. The ordinary flexion-reflex of the hind limb of the
spinal cat and dog is a reflex of this type. The end-effect of
the reflex is expressed by two groups of muscles whose con-
tractions act in opposed direction at the same joints. This
opposition is obviated in the end-effect of the reflex by the end-
effect having the form of excitatory state as regards the motor-
nerve to the flexor muscle, but suppression or withholding of
excitatory state (central inhibition) as regards the motor neurone
of the extensor. Such reflex is a reflex of double-sign, but
whereas the scratch-reflex and the eyelid-reflex, etc., are re-
flexes with successive double-sign, the flexion-reflex and reflexes
of that t3rpe, e. g.^ the crossed extension-reflex, are reflexes with
simultaneous double-sign.

The form in which this central inhibition occurs may be best
gathered from illustrative examples.

84 THE SIMPLE REFLEX [Lect.

The simple reflex mechanism examined in the swimming-
bell of Medusa gives little evidence of an arrangement for a
form of spatial co-ordination which is very prevalent in more
complex mechanisms. In many cases the body, or some part
of it, can be actively moved, not merely in one direction but
in two or more, opposed or partially opposed. The muscula-
ture is then usually divided into various discrete pieces called
** muscles." The contraction of one muscle, or set of muscles,
produces movement in one direction; the contraction of an-
other produces movement in another direction. Instances of
this are common in the limbs, neck, tail, etc., of Vertebrates
and Arthropods.

Reflex co-ordination makes separate muscles whose contrac-
tions act harmoniously, e. g, on a lever, contract together, although
at separate places, so that they assist toward the same end. In
other words, it excites synergic muscles. But it in many cases
does more than that. Where two muscles would antagonize
each other's action the reflex-arc, instead of activating merely
one of the two, causes when it activates the one depression of
the activity (tonic or rhythmic contraction) of the other. The
latter is an inhibitory eflect.

Classical examples of inhibition are those of the vagus nerve
on the heart, and of the corda tympani on the blood-vessels of
the submaxillary region. In these cases the stimulation of the
distal end of a peripheral nerve quells the existing contraction
of the muscles of the heart and blood-vessel respectively.
When the submaxillary gland is called into activity reflexly,
depression of the tonic contraction of the muscular coat of
its arteries accompanies the heightened secretory activity of
gland cells simultaneously evoked. The two reflex actions— the
one depressing the activity of one tissue, the other heightening
that of the other tissue — are mutually co-operative, and are
combined in the one reflex action, and are instances of a reflex
co-ordination quite comparable with that in which one muscle
of an antagonistic couple is thrown out of action when the other
is brought into action. And as in this case, so in some cases
of mutual co-operation of inhibition with pressor action in the

Ill] INHIBITION IN REFLEXES 85

nervous regulation of antagonistic muscles, the inhibition is
peripheral; that is to say, stimulation of the distal piece of the
divided peripheral nerve itself suffices to produce it Instances
of this occur in the claw of Astacus, and in the muscles opening
the shell of the bivalve Anodon. In Astacus, as is well known,
(Richet,*^ Biedermann,^^ Piotrowski ^^) stimulation of the distal
end of the cut peripheral nerve causes, under suitable condi-
tions, relaxation of the closing muscle at the same time as
contraction of the opening muscle. This is comparable with
the stimulation of the distal end of the cut corda tympanit which
produces relaxation of the muscular coat of the arteries of the
submaxillary gland at the same time as it causes secretion by
the gland cells.

The muscles of the claw of Astacus are striate, and the case
is interesting as one in which the co-ordination of action of two
antagonistic muscles of skeletal type is effected by peripheral
inhibition of one through the same nerve-trunk that induces
active contraction of the other. But the similar co-ordination
in the taxis of the skeletal musculature of vertebrates exerts
its inhibition not at the periphery but in the nerve-centres. It
occurs within the gray matter of the central nervous system.

When the spinal cord has been transected headward of the
lumbar region, reflex movements of the hind limb can, after
the period of shock has passed, be studied with much uniformity
of result. Electric stimuli applied to the skin of the limb,
especially of the foot, evoke practically uniformly a drawing up
of the limb. This flexion-reflex, as presented by the spinal
dog, consists in flexion at knee, hip, and ankle.

The affierent fibres from each even small area of the skin of
the foot do not enter together as a tiny group into the spinal
cord in any single filament of a single afferent root, but scatter
and make their entrance into the cord via a number of root-
lets,^* belonging not merely to one but to two or even three
adjacent afferent spinal roots. These afferent fibres having
entered the cord, severally subdivide in the manner well known
since the researches of Nansen, Ramon, Van Gehuchten, v.
Lenhossek; and others ; and their collaterals and terminals must»

86 THE SIMPLE REFLEX [Lect.

as it were, seek out the motor cells of the above-cited flexor
muscles, and, as it might appear from the above evidence, leave
the motor cells of other muscles, for instance, of the extensors,
alone. Increase of intensity of the stimulation of the plantar
skin does not in my experience make the spinal reflex action
flow over, so to say, from the flexor muscles to the extensors.
As the strength of the stimulus is increased from minimal, the
number of the flexor muscles obviously thrown into action in
the limb is increased, and the reaction irradiates to other regions
of the body ; for instance, to the extensor muscles of the contra-
lateral hind leg. In the muscles already implicated in the
weaker response the contraction becomes, as the stimulus is in-
creased, stronger, but I have not found it involve the muscles,
causing extension of the homolateral hind limb itself. This
flexor-reflex of the limb therefore appears, although able to
excite to various degrees of activity the flexor musculature of
the limb, unable to excite the extensor musculature.

It would be a mistake, however, to suppose that it is without
any direct influence on the latter musculature. It might appear
from the statement that the distribution of the aflerent con-
ductors of the reflex was to the motor neurones of flexion only,
and not to those of the extensor muscles. But the motor
neurones of the extensor muscles are not inaccessible to im-
pulses arriving by this afferent path. On the contrary, they can
be shown to be easily and habitually accessible to them.

To examine this we may turn to the "knee-jerk," and to
the tonus of the extensor muscles of the knee. In the spinal
animal, for instance in the dog and cat, after transection of the
spinal cord in the thoracic region, it is easy to satisfy one's self
that, after the shock has passed off* the extensor muscles of the
knee still possess considerable tonus. The spinal tonus is reflex,
and it has been shown ^^ that in the crureus and vastus medialis
muscles of the cat, the reflex tonus of those muscles is traceable
to afferent nerves arising in those very muscles themselves.

The reflex-arc through which the tonus is produced and
maintained arises in those muscles themselves and returns to
them again. The knee-jerk is easily elicited in the spinal cat

Ill] RECIPROCAL INHIBITION 87

and dog. The muscles which contract when the patellar tendon
is struck are in these animals the vastus medialis and crureusP^
The knee-jerk seems, however, only obtainable in them when
their reflex spinal tonus is present Its briskness varies pari
passu with the degree of this tonus. Severance of the affer-
ent nerves of these muscles destroys their tonus, and renders at
the same time the knee-jerk inelicitable, just as also does the
severance of their motor-nerves.

The knee-jerk is, therefore, like the spinal tonus itself, de-
pendent on the integrity of the reflex spinal arc of the muscles.
But it is customary to regard the knee-jerk not as a reflex action
(Westphal, Waller, and others) ; hence it is termed " knee-
phenomenon," '* knee-jerk," etc. The main grround for denying
its claim to be really reflex is that its latent period is shorter
than that of other indubitable reflexes. The latency for the
knee-jerk has been shown (Waller,^** Gotch,^®^ and others) to
be about 10 a-, whereas the shortest latency found by Exner* for
reflex eyelid-closure was 45 c and by Fr. Franck ^" for a spinal
reflex about 170*. The latency for the knee-jerk is but little
longer than that for direct excitation of the extensor muscle
itself.

If we regard the knee-jerk not as a true reflex but as a
"direct" response of the muscle, we have to suppose that the
reflex tonus of the muscle, which is admittedly a conditio sine
qua non for the jerk, so raises the direct excitability of the
muscle that the muscle responds by a contraction to a sudden
slight stretch of itself due to a tap on its tendon. No experi-
menter has, however, satisfactorily succeeded by artificial stimula-
tion of the motor-nerve in similarly raising the direct excitability
of the muscle. Moreover, Gotch^®^ found the muscle in its
state of tonus gave no other indication of increased excitability
than simply that it then yielded " the jerk." It has been urged
against the reflex nature of the jerk that the contraction given
by the muscle to the jerk is a simple twitch. The "jerk " con-
traction lasts no longer, or hardly longer, than the twitch given
by the muscle in response to a single stimulus, e,g,^ an induction
shock. All reflex contractions are usually considered as tetanic.

88 THE SIMPLE REFLEX [Lect.

That is in the main doubtless true. It is what might be inferred
from the great part played by summation of stimuli in the elici-
tation of reflexes. Yet the extensor-thrust reflex, which is
undoubtedly a true reflex, appears on measurement (p. 67) to
be as brief as the knee-jerk. Its time-relations have been re-
ferred to. It is interesting that this brief-lasting reflex also has,
as has the knee-jerk itself, the extensor muscles of the hind
limb for its seat of expression. The mere brevity of the period
of contraction of the knee-jerk is therefore no good evidence
that it is not reflex.

The knee-jerk, whether reflex or not, furnishes, since it
is an index of the reflex tonus of the extensor muscles, a
gauge for the eflect, if any, exerted by the flexion-reflex
on the extensor muscles of the limb. It was said above
that the extensors are not thrown into contraction by this
flexion-reflex. The reflex reaction may therefore either be
neutral to them and leave them and their condition untouched,
or it may inhibit them and depress their reflex activity, even if
that activity have at the time only the form of tonus.

If the hamstring muscles (flexors of the knee) be separated
from their attachments at their distal (knee) end, and then while
the knee joint is passively held in approximate or full extension
the flexor-reflex be elicited, e. g. by electric stimulation of the
foot, the extensor muscles above the knee are easily felt by pal-
pation to at once lose their tonus and relax.^ At the same
moment the exposed and freed flexor muscles are seen to enter
contraction. That is to say, the same exciting stimulus that
reflexly throws the flexors into contraction interrupts reflexly
the reflex tonus of the extensor muscles. If the knee-jerk be
elicited at regular short intervals, signalled for instance by a
metronome, and while it is in progress the flexor-reflex be elic-
ited after the flexor muscles have been detached from their
knee attachments and the knee thus left free, the knee-jerk
is found inelicitable or much diminished directly the reflex
contraction of the hamstring muscles sets in (Fig. 29). This
inhibition of the jerk sometimes seems to set in even before
the reflex contraction of the flexors is apparent. It occurs

UI]

INHIBITION OF KNEE-JERK

sometimes when the stimulas is not even strong enough to evoke
obvious contraction of the flexors. In the ** flexion-reflex,**
therefore, the reflex excitation of the flexor muscles is accom-
panied by reflex inhibition of the antagonistic extensor muscles
both as regards their reflex tonus which is in progress when the
flexor-reflex is excitedi and as regards their response to a stim-
ulus (tap on tendon or muscle) that otherwise excites them*

TuivnM 99. — Tricmg from prepttiliiNi of the ateiisor nnucki of the knee, recordiBK 1
Mfiei of knee^jcdts elidled Mt each dtemaee beat of a metnmome. Weak ^radixatioii of
the central cad of tlae bamsCriiis oerve wss applied dnrin; the time otarked by the
ugtal line below. The toons of the ex tensor preparation at onoe fell^ and with it the
knofr-jeik wati tempocarily aholished. After ceisation of the inhibiting stimulus the tonus
and the k]tee^)erk quickly retiirned, and the latter becune more brisk than prerious to the
The lowest line marks the time in seconds.

A corresponding reaction is seen also after ablation of the
cerebral hemispheres and thalamencephalon. After removal
of those organs, there ensues " decerebrate rigidity/' **•» ^® One
feature of this condition is a heightened tonus of the extensor
muscles of the knee. The knee is maintained rigidly extended.
At the same time the knee-jerk is elicitable in unusual degree.
When the knee is under these circumstances freed from the
flexor muscles, and the flexor-reflex is then induced by appro-
priate excitation, e^^^, of the plantar skin, the knee joint at once
drops loose, and if the knee-jerk be tested, it is found to be
ineltci table, or elicitable only very faintly (Fig. 29).

Similarly, if instead of the knee-jerk or the reflex rigidity
of the decerebrate animal, we take the reflex termed the ex-

90 ^ THE SIMPLE REFLEX [Lect.

tensor-thrust as a guide to the condition of the extensor arcs
during the flexion-reflex, we get similar evidence that those arcs
are temporarily out of action. While the flexion-reflex is in
progress the extensor-thrust is less elicitable. If the flexion-
reflex is quite weak, the extensor-thrust can be obtained and
breaks through it ; but it cannot if the flexion-reflex be of fair
or of considerable intensity. The reflex called the extensor-
thrust is an extremely powerful one ; it can in the spinal dog
lift the whole body from the g^round and push it forward.
Yet none of the devices normally evoking it can elicit it during
a fair flexion-reflex. It becomes elicitable again when the
flexion-reflex is over.

It seems, therefore, that in the flexion-reflex and in the other
above-mentioned reflexes an inhibitory process is part and
parcel of the reflex reaction, so that the inhibition goes on side
by side with excitation of other muscles opposed to those which
are inhibited. This view, that the inhibition process in these
reflexes is a simultaneous counterpart to the excitatory, is
supported by the following evidence from the flexion-reflex.

A salient feature of this reflex is flexion at the knee. For
comparison of the inhibition and excitation respectively, both
hind limbs are taken and so prepared that in one leg only the
knee flexors can act, in the other leg only the knee-extensors.
The stimuli to provoke the reflex are applied either to sym-
metrical skin points or to symmetrical aflerent nerves at, as far
as practicable, symmetrical places in their course. For compari-
son, the stimuli are made as far as possible equal on the two
sides. This being arranged, certain characteristic features of the
reflex have been examined on the two sides respectively.

(a) The flexion-reflex has a " receptive skin-field " which
though extensive is characteristic for it Examined by the
above preparation the skin-field whence the excitation (con-
traction) is elicitable and that whence the inhibition is elicitable
has proved in my observations to be one and the same. Thus :
stigmatic unipolar faradization of a point in the skin of a right
pedal digit provokes in the homonymous limb contraction of the
flexors of the knee, and similar stimulation of the correspond-

Ill] RECIPROCAL INHIBITION 91

ing left digit provokes in its own limb inhibition of the extensors
of the knee. Again, similar stimulation of the skin of the fore
foot (in my experience that of the crossed fore foot acts more
readily than that of the homonymous) induces excitation (con-
traction) of the flexors of the crossed knee ; and the correspond-
ing skin-region of the opposite fore limb induces inhibition
(relaxation) of the extensors of the knee contralateral to it

(/9) Turning to stimuli other than electrical, it is not, as I
have pointed out, every form of stimulus that, when applied
within the skin-field appropriate for the direct flexion>reflex, can
excite it The kinds of skin-stimuli which excite it are those
which may be termed '* nocuous," *" e, g., a prick, strong squeeze,
harmful heat (the heat-beam), and chemical agents. Touches,
innocuous pressures, rubbing, etc., though eflective for various
reflexes, e. ^., for the extensor-thrust, scratch-reflex, pinna-
reflex, etc., do not in my experience excite this reflex. The
stimuli which do excite it, for instance, from the planta^ excite,
when applied on the side where the flexor muscles alone remain
intact, contraction of those muscles, and when applied corre-
spondingly on the opposite side, where the extensors alone
remain intact, inhibit them (relaxation).

(7) The nerve-twig, similar to that which under faradization
on the " flexors " side excites the flexors (contraction) when
faradized on the "extensors" side inhibits the extensors (re-
laxation). This comparison has been made not only with
skin nerves, but with muscular nerves, notably with the nerves
of the hamstring muscles and of the gastrocnemius.

(S) The flexion-reflex, although it exhibits well the potency
of summation of successive stimuli as a factor in its initia-
tion, diflers in my experience from various other reflexes,
e, g.^ extensor-thrust, scratch-reflex, pinna-reflex, in being elicit-
able fairly easily by a single-induction shock. The shock
may be applied either to the skin in the receptive skin-
fleld of the reflex or to an appropriate afferent nerve either
cutaneous or muscular. When this is done in the prepared
limbs the single-induction shock applied on the " flexors " side
excites a brief reflex contraction of those muscles, correspond-

Lw? iiavAi mm ¥

S!AilfO?'J UNiVERSITY
fi*!.0 M.TO. Oi.W^^«W^

92 THE SIMPLE REFLEX [Lect.

ingly applied on the " extensors " side it provokes a brief reflex
inhibition of those muscles.

(e) The flexion-reflex, unlike extensor-thrust, pinna-reflex,
etc, can be well evoked in my experience by make or break
of a galvanic current This make or break reflex is shown
in the " extensor " preparation by inhibition, just as it is shown
in the "flexor" preparation by contraction. With suitable
strength of stimulus the break of a descending current is more
eflective for the reflex inhibition than the make, and vice versa
for an ascending current, just as with contraction. The flexion-
reflex can also to a much greater extent than can the scratch-
reflex be maintained by passage of the constant current In
this respect it resembles the vasomotor and respiratory reflexes
examined by Griitzner,^ and by Langendorfl* and Oldag,^*^
and also the sensual reaction which similar stimulation excites
in ourselves — a point of interest when the connection between
nociceptive reflexes and dolorous sensation is remembered.
When the constant current is thus applied to the limb in which
the extensors have been prepared, inhibition proceeds in them
as does contraction in the flexors when that current is similarly
applied to the limb in which the flexors have been prepared.

({) The latent time of the flexion-reflex is short This
feature is revealed in the inhibition of the extensors just as in
the contraction of the flexors. Great differences of latency
in the flexion-reflex as in other reflexes can be obtained by,
apart from variance in intrinsic condition of the reflex prepara-
tion, variance in the external stimuli in intensity, suddenness,
frequency of repetition, etc. The effect of such variations is the
same in kind, and, in my experience, in extent, when tested by
the reflex inhibition as when tested by the reflex contraction.
Thus, with strong stimuli I have found as short a latency as
32 <r for the inhibition, which is slightly shorter than the shortest
for contraction under like circumstances that I have yet met
with. With weak stimuli I have occasionally met with a latency
as long as 400 <r for each effect

(1;) A good criterion of comparison between the reflex in-
hibition and the reflex contraction in the flexion-reflex under

Ill] RECIPROCAL INHIBITION 93

excitation by an intermittent stimulus is the number of stimuli
summed for initiation of the reflex as exhibited on the one hand
in contraction of the flexors, on the other hand in relaxation of
the extensors. The number of successive single stimuli summed
for the initiation is less as their individual intensity is greater.^

When the summation is compared in the same reflex prepa-
ration, in the reflex exhibited las inhibition (relaxation) in the
knee-extensors of one limb and in the reflex exhibited as con-
traction in the knee-flexors of the other limb, good agreement
is found ; the number has been often actually the same, though
the observations are made alternately, flrst one on one limb, then
one on the other limb. Figs. 30 A and 30 B, and 31 A and 31 B
are such psurs, and illustrate the kind of agrreement.

(0) The course of the flexion-reflex as shown in myo-
grams differs much from that of certain other reflexes of the
limb, notably from the extensor-thrust and from the scratch-
reflex. Its duration follows more closely that of the eliciting
stimulus. If the stimulus is quite brief and not intense the
myogram shows but a short continuance of the development
of the eflSect after the external stimulus itself has ceased. The
flexion-reflex by adjustment of the intensity of the stimulus
can be graded as to its amplitude. This grading is seen not
only as a grading of the amplitude of contraction of the flexors
when the stimulus is applied to the limb with intact knee-
flexors, but as a grading of the amplitude of relaxation when
the stimulus is applied to the limb with intact knee-extensors.

These correspondences support the view that the reflex
inhibition (relaxation) and the reflex excitation (contraction)
are part and parcel of one and the same reflex reaction ; and that
although opposite in direction they are co-ordinate reciprocal
factors in one united response.

In the crossed extension-reflex this " reciprocal innervation "
b seen conversely inhibiting the flexors while causing contrac-
tion of the extensors. This reflex is well excited by stimulation
of the opposite planta. The myog^raph lever recording the
contraction of one of the hamstring muscles, isolated to sample
the group, is then seen to register a quick relaxation interrupt-

ing the reflex contraction that until then had been in progress
(Fig. 32)* The speed with which the reflex inhibition occurs
and is accomplished is much the same as that which reflex con-
traction itself exhibits. It hardly seeras slower. But it is often
noticeable that the relaxation thus induced in the contraction
does not reduce the contraction to zero (Fig. 32). The relaxa-
tion ensues down to another grade of contraction, at which
grade the inhibition often continues to hold it; at least, the
muscle continues to remain at that length. In such cases the
contraction is reduced suddenly from a high level of intensity
to a lower level, but a remainder of contraction persists- It
may be that this lower grade represents another functional act
in which the muscle is simply adjuvant towards steadying the
levers for other muscles which replace itself in its previous rdle

Ill] RECIPROCAL INHIBITION 99

of principal actor. The condition under which I have most
frequently met it has been when the exposed and freed
tendon of the semitendinosus (dog) has been attached to the
myograph.

It is interesting that when (Fig. 33) the inhibiting stimulus
is strong the relaxation of the extensor muscles is actually to
a point beyond their initial length obtaining at the time the
"crossed extension-reflex" began. The pre-existent" decere-
brate" tonus is inhibited as well as the intercurrent reflex.
The relaxation is indeed down to, as I expressed it in one of
my earlier notes,*^ the post mortem length of the muscle. The
relaxation, if the crossed-reflex stimulus continues, is rapidly
recovered from, and the interrupted reflex reasserts itself
(Figs. 33 and 34).

Concordantly with these results examination with the myo-
graph of the contractions of the pretibial and post-tibial muscles
of the fr<^ ^^ during alternate flexor and extensor strokes of the
hind limb shows in many cases, though not in all, that the con-
tractions of the two antagonistic muscles are not synchronous,
but are conversely timed. The contraction of the pretibial
musde breaks down just as that of the post-tibial ensues, and
the post-tibial relaxes just as the pretibial contracts.

An early noted and in various ways typical example of this
r61e of reflex "inhibition " was that discovered by E. Hering**
and J. Brener** (1868) in the " self-regulating " respiratory vagus
action. Distension of the lung by exciting aflerent flbres in the
pulmonary vagus inhibits inspiration and excites expiration. I
reverted to this as a fundamental instance in my flrst note ^^ on
the subject If we regard the heart and ring-musculature of the
arteries as two antagonistic muscles, v. Cyon's ^ still earlier dis-
covery (1866) that the aflerent nerve of the heart, and aorta
(A. Tschermak and Koster)**^ — from this point of view, a
tendon of the heart muscle — evokes reflex inhibition of the
arterial ring musculature, is another instance. The sugges-
tiveness of these facts for the co-ordination of skeletal muscles
was not recognised generally. But Meltzer, the discoverer with
Kronecker* of the rdle of inhibition in normal deglutition,

lOO THE SIMPLE REFLEX [Lect.

wrote in 1883,^^ "Of a purposeful arrangement wc could ex-
pect that a nerve, the stimulation of which causes flexion,
ought to contain also inhibitory fibres for the extensors. Now
such an arrangement is indeed present — at least in the re-
spiratory mechanism. Of the superior laryngeal nerve, of the
second branch of the trigeminus, and of the splanchnics, we
know that stimulation of their central end causes inhibition of
the inspiratory and contraction of the expiratory muscles."

Now in the case of the skeletal muscles of the mammalian
limb, no efferent nerve-fibres appear to be supplied to them
which under stimulation produce inhibition of their contraction.
Such have been sought for by various observers, but without
success. I have myself looked for them and obtained no un-
equivocal evidence of their existence. Moreover, Verwom*"^
has shown that during the inhibitory relaxation produced by
the reflex induced from the nerve of the antagonistic muscles
the excitability of the relaxed muscle and its motor-nerve to
electrical stimuli remains undiminished.

Moreover, in the condition of decerebrate rigidity, when the
elbow is being kept in extension by the heightened tonic action
of the extensor muscles, their contraction can be inhibited not
only by stimulation of the crossed hind foot, but by direct
electrical stimulation of a point in the lateral column of the
transected spinal cord in the hind thoracic region, as has been
shown by A. Frohlich and myself.^^ The inhibition reflexly
produced has therefore its seat in the spinal part of the reflex-
arcs. It is therefore a central inhibition.

This central inhibition appears more than equivalent to
merely arresting the play of an excited afferent channel upon

Figure 33 (opposite). — Myograph record of reflex contractionof extensor of knee interrupted
by a reflex inhibition (relaxation). The reflex contraction was induced by stimulation (uni-
polar faradization) of the skin of the opposite foot: this stimulation was applied during
the time marked by the lower signal ; its moments of conunencement and ending are
marked by abscissae on the myogram. Towards the height of the reflex contraction a
brief stimulation (unipolar faradization) was applied to the skin of the foot homonymous
with the knee extensor yielding the myogram : the duration of this inhibiting stimulus is
marked by the upper signal. The knee extensor at outset was in some tonic contraction
due to '* decerebrate rigidity." The reflex inhibition relaxes this in addition to inhibit-
ing the current reflex from the crossed foot. Time is marked below in fifths of seconds.

the motor centre. Were that all, the phenomenon should re-
semble the effect of suddenly stopping the stimulation of the
afferent nerve causing the reflex. What happens is often not
like that ; the arrest is more rapid. The ** after-discharge,"
whatever its seat, can be at once arrested by the inhibition
(Fig. 35). The ** after-discharge " of a centre^ with its con-
comitant persistence of contraction of muscles, might well be
disadvantageous to the organism* That it is rapidly arrested

Ill]

RECIPROCAL INHIBITION

the inhibitory side of a succeeding reflex, is an ac
irhich facilitates the successive interchange of reflex
libitjon can arrest some fonns of clonic spasm arisiis
Kperimentatiofi (Fig. 56),
The motor neurones of the flexor muscles of the h
be exerted to the clonic discharge characteristi"
scratch-reflex at a time when the flexion-reflex is
bom employing them. When the scratch-reflex is in

FtcuiLK 35.— Myrog ftpb records of reflex contractiotLa of tbe extensor of the kni
bote ** tat. The exciting stimulus was, in the ofatervatioo reproduced on tJ
figure, i fariel compression -^ lasting less than a second — ofm digit of the
foot. After this stimalm had been given and discontinued^ and while the aH
of the rcflen was still in progress, the proxima] end of a branch of the Kven
nerve was stimutaCed by faradtzation for about a quarter of a second. The
feahibitiA^ stimulus is marked by the signal. The reflex afto'-discharge isi
bcBi at otice inhibited and in this case not to have returned.

The obserration reproduced on the right was from the same experiment, bn
tbe stimulation eacdtiog tbe leflex contiactian was faradization of the proxiq
twig ol the internal lapheiious of the contralateral leg. This stimulation I
two fifths of 1 feoood or fess. 1t» ccsaatioii was quickly succeeded by faradil
proximal cad of a branch of the serered hamstring nerve as in the previous
The ligBal marks the time of this inhtbitiog stimulation. The after-discharg
tiactioo rdlex is cot short as before. Time la marked bebw in fifths of

104

THE SIMPLE REFLEX

«;i',r

i>.!

Fir.URE 36. — Myogram of convulsive twitchitig oi semttevdin^ius in a " spinal " dog. The '
spasms are reduced and tcmponuily suspended by stimulation (faradization) of the proxi*
mal end of a branch of the internal saphenous nerve of the contraJateral leg. The lime
of application of the inhibiting stimulus b shown on the signal line below* Time is
marked above in seconds.

it is more difficult to excite a "flexion-reflex," and vice verstu^
One reflex seems to be precluded from acting on a motor
neurone at a time when another and different reflex is employ-
ing it*^ The preclusion of the motor neurone from one reflex
while it is still left open to it to respond to other reflexes ap-
pears to be one of the services of inhibition to the organism.
The motor neurone itself seems not the actual seat of the in-

Ill]

RECIPROCAL INHIBITION

105

hibition, for if so, it would be inhibited for all reflexes; un-
less the motor neurone is functionaliy divisible* and one part
of it, #. ^., one set of dendrites^ can be inhibited at a time when
another is not* The seat of the inhibition appears, therefore,
with some likelihood, to lie neither in the afferent neurone
proper nor in the efferent neurone proper, but in an internun-
cial mechanism — synapse or neurone — between them. I say
" neurone proper,*' meaning to exclude from that term the
synapse, although in a synapse the neurone terminals are
included.

The striking correspondence observed (^zk j.) between the
reflex inhibition and the reflex contraction, when examined in
one and the same type-reflex, allows the inference that the
nerve-fibres from the receptive field of the reflex each divide
in the spinal cord into end-branches (^.^., collaterals), one set
of which, when the nerve-fibre is active, produces excitation,
while another set, when the nerve-fibre is active, produces inhi-
bition.*^* *** The single afferent nerve-fibre would therefore in
regard to one set of its terminal branches be specifically excitor,
and in regard to another set of its central endings be specifically
inhibitory. It would, in this respect, be duplex centrally (Fig. 37).
There is analogy between the structural arrangement for reflex
reciprocal innervation and that of Astacus claw, if it be sup-
posed that the individual nerve-fibres of the crayfish-claw prepa-
ration dichotomise, one division of the nerve-fibre passing to
the closing muscle, the other to the opening muscle; so that
one division of the fibre exerts the excitor action, the other
the well-known inhibitory, studied by Richet,* Biedermann/^
Piotrowski,"^ and others.

In denoting one set of central terminations of an afferent arc
•* specifically inhibitory^* it is here meant that by no mere change
in intensity or mode of stimolation can they be brought to yield
any other effect than inhibition- But the fact that stimulation
of a single set of afferent arcs, namely a single small afferent
nerve, excites frequently a reflex movement of alternating direc-
tion in which, for instance at the knee, extension succeeds pri-
mary flexion, shows that a change of internal conditions may

io6 THE SIMPLE REFLEX [Lect.

presumably convert an intraspinal connection that under the
primary conditions is inhibitory into one that under later super-
vening conditions becomes excitatory. The fact that under
certain forms of cerebral action true antagonistic muscles can
be thrown synchronously into contraction, points to the same
limitation of the term " specific " in this connection. Further,
there is the intraspinal action of strychnine.

There is the long recognized fact that under strychnine
practically all the skeletal muscles of the body may be reflexly
thrown into contraction simultaneously, and this is obviously
inclusive of, and was proved for, antagonistic muscles.^* Evi-
dently strychnine in some way must alter or obscure reciprocal
innervation. I have furnished (1892) tracings showing that the
pretibial and post-tibial muscles of the frog, although in normal
reflex movements so frequently exhibiting concurrent contrac-
tion and relaxation in the two groups reciprocally, under
strychnine reveal in the double myogram perfectly synchronous
contraction in both groups.^

Such a result may be explicable in several ways. In order
to discover what the nature of the change wrought by strychnine
really is, there have to be fulflUed in the test experiments cer-
tain conditions which not every preparation of antagonistic
muscles can supply. Muscles acting over two joints are to be
avoided in such a test Thus the gastrocnemius of the frog ex-
tends the ankle but flexes the knee ; it antagonizes the action
of the pretibial muscles which flex the ankle, but since flexion
of the knee so commonly accompanies flexion of the ankle, it
is synergic with the pretibial muscles in the great flexion-reflex
that draws up the limb. If it acts synchronously with pretibial
muscles under strychnine, we are still left in a dilemma as to
whether the co-ordinate reciprocal action at the ankle is essen-
tially destroyed, or whether a reflex attempt to flex the knee
has not been simply added to it under a lowering of the intra-
spinal resistances. And this dilemma is the greater in that the
afferent nerves and surfaces used for exciting reflexes contain
admixed aff'erent channels, some exciting contraction in one
group of the opposed muscles, and some exciting contraction in

Ill] ACTION OF STRYCHNINE 107

the other. Thus in the afferent nerves from the foot, both in the
dog*^ and the frog,** there are commingled with fibres which
excite the flexor muscles those which excite the extensor muscles
— witness the extensor-thrust and flexion-reflex, both elicitable
from the dog's foot, and Baglioni's extension-reflex of the leg
and the flexion-reflex, both elicitable from the frog's leg. That
the extensor muscles of the limb should under strychnine be
thrown into contraction s3mchronously with the flexors in these
cases might be due either to the two reflexes being elicited
together when spinal resistance has been lowered, or to a con-
version of the inhibition part of one reflex into an excitation.
And in this latter case it is still left undecided whether the
extension under strychnine is due to prepotent extensor-reflex
with its accompanying flexor inhibition changed into excitation,
or whether it is the flexion-reflex which is changed conversely.

On similar grounds the " spontaneous " convulsions due to
strychnine afford no deeper insight into the problem. These
" spontaneous " convulsions are really reflex (Stannius, CL
Bernard, H. E. Hering, and others) in the sense that they
originate in the afferent arcs ; and in the convulsive movements
antagonistic muscles contract simultaneously. But the dif!iculty
here again is, that the reflex source may, and probably does,
operate in many afferent arcs concurrently. Some of these arcs
excite extensor muscles normally, while others excite flexors.
The simultaneous contraction of both flexors and extensors
might thus be naturally explicable by lowered spinal resistance,
both sets of reflexes being equally induced together, or the
explanation might be of an alternative kind, such as suggested
above. On the former view the reciprocal innervation of antag-
onistic muscles would merely be obscured by a simultaneous
double reflex; on the latter a more profound alteration would
have taken place. The occurrence and form of the convulsion
fail to decide among these possibilities.

Conditions for determining the nature of the action that really
occurs seem offered, however, in certain instances. Thus, in the
hind limb of the cat we have two afferent nerves which never,
under any normal conditions *^ in my experience, yield as their

io8

THE SIMPLE REFLEX

[Lect.

primary reflex in the vasH-crureus muscle any action but relaxa-
tion ;' in other words, they, without exception, produce reflex
inhibition of that muscle. To suppose that these nerves contain
aflerent flbres which evoke reflexly at the knee any action other

Figure 37. — Diagram indicating connections and actions of two afferent spinal root-cdls, «
and a', in rtgud to their reflex influence on the extensor and flexor muscles of the two
knees, a, root-cell afferent from skin below knee; a', root-cell afferent from flexor
muscle of knee, /. f., in hamstring nerve ; c and «', efferent neurones to the extensor
muscles of the knee, left and right ; 6 and h\ efferent neurones to the flexor muscles ; E
and E\ extensor muscles ; F and F', flexor muscles. The '* schalt-zellen " (v. Monakow)
probable between the afferent and efferent root<elIs are for simplicity omitted. The sign
-h indicates that at the synapse which it marks the afferent fibre « (and a') exdtes the
motor neurone to discharging activity, whereas the sign — indicates that at the synapse
which it marks the afferent fibre a (and a*) inhibits the discharging activity of the motor
neurones. The effect of strychnine and of tetanus toxin is to convert the minus sign into
//«j sign.

Ill] ACTION OF STRYCHNINE 109

than flexion would be mere hypothesis. These two nerves are
the internal saphenous in its course below the knee, and the
hamstring nerve, coming from the flexor muscles of the knee.
Further, the vasti-crureus is a single-joint muscle, and unlike
the rest of the quadriceps extensor of the thigh is not a flexor
of the hip ; therefore contraction in it cannot mean merely its
participation in the synergy of the flexion-reflex itself, which
includes flexion at the hip. A reflex preparation suited for ex-
amining the action of strychnine on reciprocal innervation can,
therefore, be obtained in the hind limb by severing, in the de-
cerebrate animal for instance, the following nerves : the external
popliteal, the internal popliteal, the obturator and pudic in the
pelvis, the superior gluteal, the external and cutaneous divisions
of the anterior crural and the hamstring nerve. The last named
is ligated and cut close to its entrance in the muscles, so that
its central end can be stimulated. A branch of the internal
saphenous nerve below the knee is also prepared for stimulation
of its central end. When this is done it is found that no change
in intensity or other conditions of excitation of the aflerent
nerve ever provokes an3^hing but inhibition of the extensor
of the knee, but a small dose of strychnine at once transmutes
the inhibitory effect into an excitation effect*^ Reflex con-
traction is obtained in place of reflex relaxation. If small doses
are carefully graded, it is possible to see a state in which the
reflex relaxation is diminished but is not replaced by excita-
tion. This phenomenon shows well how little competent is the
view of lowered spinal resistance to really explain the action of
strychnine ; for at this stage the stimulated arc that normally
acts on the extensor muscle by inhibition is less able to affect it
than before, so that on the spinal resistance view the resistance
at this stage is actually heightened.

A similar conversion of inhibitory effect into excitatory
is produced more gradually but not less potently by tetanus
toxin.*^

This conversion sets in before and under smaller doses of
strychnine or toxin than are required to produce the convulsive
seizures characteristic of strychnine poisoning, or general tetanus.

no THE SIMPLE REFLEX [Lect.

The transformation of effect by strychnine holds good not
only for the nerves above mentioned but for skin-stimuli, and also
for those skin points remote from the hind limb itself, which pro-
voke reflex inhibition of the test muscle. For instance, in the
case of the knee-extensor as test muscle, the skin of the fore paws.

The conversion of inhibitory effect into excitation effect by
strychnine is more easily obtained in the case of some nerves
than of others. In the instances of the nerves above mentioned
the conversion is least facile, i. e.y requires larger doses or longer
time for development, in the case of the hamstring nerve, than in
the others. The inhibitory effect belonging to that nerve is
readily lessened by the strychnine, but its actual replacement
by excitation effect, e.g., contraction of knee-extensor, not only
requires larger doses of strychnine, but is even then phasic rather
than continuous. When this nerve is tested by stimulation at
regular short intervals during one of these phasic periods, it can
be seen that, starting from the phase in which it still evokes in-
hibition little, or perhaps not at all, less obviously than in the
normal state, its inhibitory effect then becomes progressively less,
until it is replaced by excitation effect (contraction), at first mild,
later violent. This periodic phase will repeat itself many times.

The conversion of inhibition effect as thus tested on the
knee-extensor might be attributable to the afferent nerves stimu-
lated containing two kinds of afferent fibres admixed, one kind
causing reflex contraction of the muscle, the other kind reflex
inhibition. Strychnine might, by augmenting the action of the
former or by depressing the action of the latter, change the
effect of stimulation of the mixed nerve. But the latter fibres
would be expected to be associated in their action with — or, as
•urged above, to be even the self-same fibers which evoke — con-
traction of the flexor muscles. Now there is at the stage of
strychnization, at which the change of inhibitory into excitatory
effect occurs, no trace of any paralysis or even depression of
the flexor contractions. The protagonist and the antagonist
muscles are thrown together into synchronous contraction as
an effect of strychnine. This and other considerations ap-
pear to me to weigh against explaining the conversion of

in] ACTION OF STRYCHNINE iii

inhibition effect into excitation effect by the hypothesis of reflex
antagonistic sets of fibres, oppositely poisoned centrally, com-
mingled in these afferent nerves. Moreover, when a hamstring
muscle is taken as the test muscle, a similar conversion of
inhibition into excitation (contraction) by strychnine is seen
under the crossed extension-reflex. This reflex, elicitable
through the skin or various afferent nerves of the contralateral
hind limb, normally excites the knee-extensor to contraction and
inhibits the hamstrings, the knee-flexors. Under strychnine its
reflex inhibition of the hamstring muscle is converted into reflex
excitation Ccontraction) of that muscle. The observations, as
they stand at present, incline me to the inference that the action
of the alkaloid is to convert in the spinal cord the process of
inhibition — whatever that may essentially be — into the process
of excitation — whatever that may essentially be.^ The reflex
nexus was pre-existent, but the effect across it was signalized by
a different sign, namely minus prior to the strychnine or tetanus
toxin, instead otplus, as afterward (See Fig. 37).

The action of the toxin in respect to inhibition resembles
that of strychnine closely in several ways. Thus, in the stages
of the disease in which the tetanus is still " local " and manifested
in one limb, namely that (^. g., the hind limb) which received
the toxin injection, the toxin early converts into excitation the
reflex inhibition of the extensor muscles, normally obtainable
from the internal saphenous nerve, but that obtainable from the
peroneal and popliteal nerves, and from the hamstring nerve,
remains unreversed, though the strength of the inhibitory effect
of these nerves may be very distinctly less than normal. Later,
as the condition progresses, the inhibitory effect normally be-

1 From the predominance of extension as a reflex in the hind limb of the ' spinal '
frog under strychnine, flexion predominating normally, Cushny has recently (3d
Edition of Textbook of Pharmacology and Therapeutics, Philadelphia, 1903)
argued much as I did (Jouni. of Physiology, vol. xiii, 1892), also from experiments
OB the frog, that strychnine acts as a destroyer of reciprocal innenration. The
hind Umb of the frog, owing to the number of double-joint muscles, is, as said in the
text, not a preparation wherewith it seems possible to definitely test this argument
or inference. But the evidence obtained from more suitable preparations fully
bears out, as the text shows, the earlier inferences drawn by Cushny and myself
fcgarding the frog. [October, 1905. C. S. S.]

112 THE SIMPLE REFLEX [Lect.

longing to the peroneal and popliteal nerves becomes actually
reversed into excitation. Finally, even that of the hamstring
nerve itself is reversed. This is the same sequence of effect
pursued by progressive increase of dosage of strychnine.

One difference that seems apparent between the action of
the tetanus toxin and the strychnine in these observations is
that in the relatively slow progress of the tetanus it is easy to
note the stage in which the conversion of the inhibition effect
into excitation effect has occurred, while there is yet none of
that obvious lowering of the threshold of reflex reaction which
early marks the course of strychnine poisoning, and has been
drawn attention to by many observers.

In experiments on the hind limb, I have usually introduced
the toxin into the sciatic trunk well below the hamstring branch,
more rarely into the hamstring nerve as well, or alone. I have
found the inhibitory effect of the internal saphenous nerve
(stimulated in its course below the knee) converted to excitation
in forty-eight hours from the time of inoculation. In the gradual
progress of the condition, I have several times found the ham-
string nerve produce slight inhibition of the extensor if the initial
posture taken at the knee be extension, and yet produce distinct
excitation of the extensor if the initial posture taken at the knee
be flexion. This recalls the results of v. Uexkiill in Ophioglypha
and Echinus.*^

The conversion of inhibition into excitation by tetanus toxin
is demonstrable, as is that by strychnine, with the reflexes of the
fore limb as well as with those of the hind limb, and in the
** decerebrate " animal as well as in the merely " spinal. "

We can understand what havoc such a change must work in the
co-ordinative mechanisms.*^ The observed difference between
the facility with which strychnine and tetanus toxin convert the in-
hibition by the hamstring nerve into excitation, and that with which
they convert the inhibition of the other limb-nerves mentioned,
does not seem referable to a different action on muscular affer-
ents and cutaneous afferents respectively. Stimulation of the
central end of the vast<hcrureus nerve evokes normally inhibition
of the hamstrings of the opposite limb, but under strychnine it

ni] ACTION OF STRYCHNINE 113

evokes their contraction. In that case, therefore, the strychnine
converts with facility the inhibition by a muscular afferent into
excitation, just as with the skin nerves mentioned.

That strychnine and tetanus can convert a central inhibition
into an excitation, and that the various normal reflex spinal in-
hibitions show differences one from another in the ease with
which they undergo conversion into excitation makes the syn-
chronous excitation of antagonistic muscles in certain willed
actions less difficult to understand. Vasodepressor reflexes
under chloral (v. Cyon), chloroform (Bayliss),^^ etc., change
into vasoconstrictor under curare, morphia, etc. But the re-
versal does not appear to occur with equal facility in all
afferent nerves alike. It is stated to be impossible to obtain
any vascular reflex, but a depressant one from the " depressor "
nerve. This nerve, arising in the heart (v. Cyon)*^ and aorta
(Koster and A. Tschermak),^*^ may in a sense be considered
the afferent nerve of the muscle antagonistic to the ring muscu-
lature of the arteries, namely, the muscle whose tonus it reflexly
depresses. It is in that way comparable with the afferent
nerve of the hamstring muscles in relation to the extensors of
the knee. The depressor action of the hamstring nerve on
the knee-extensor seems, as just said, in my experience, particu-
larly resistant to conversion from inhibition into excitation by
strychnine.

In examples of reciprocal innervation drawn from lowlier
organisms and visceral organs we find the inhibition a periph-
eral phenomenon, that is, with its seat outside the central nervous
system. On the other hand, in examples drawn from- higher
organisms and skeletal movements, the inhibition is a central
phenomenon, e,g.^ intraspinal. The same considerations which
traced the line of adaptation in placing the seat of the refrac-
tory phase of spinal reflexes intraspinally between the ending
of receptive neurone and the commencement of motor neurone,
apply here to central inhibition. The significance of the cen-
tndization of these processes of refractory phase and reciprocal
inhibition seems the same as we may infer for the centrality of
the central nervous system itself (^vide infra, Lecture IX).

BtO ALTO. CMJI?0«MK

114 INTERACTION BETWEEN REFLEXES [Lect.

LECTURE IV

INTERACTION BETWEEN REFLEXES

Argument: The "simple reflex" a convenient but artificial abstraction.
Compounding of reflexes. The principle of the common path. Rela-
tive aperiodicity of the final common path. Afferent arcs which use
the same final common path to different effect have successive but
not simultaneous use of it " Allied " reflexes. Allied reflexes act
harmoniously^ are capable of simultaneous combination, and in many
cases reinforce one another's action on the final common path.
"Antagonistic" reflexes. Alliance or coalition occurs between (i)
individual reflexes belonging to the same " type-reflex," (2) certain
reflexes originated by receptors of different species but situate in the
same region of surface, (3) certain reflexes belonging to propriocep-
tive organs secondarily excited by reflexes initiated at the body-
surface (the three fields of reception, extero-ceptive, intero-ceptive,
and proprio-ceptive), (4) certain reflexes initiated fix)m widely separate
but functionally interconnected body-regions. Alliance between re-
flexes exemplified in inhibitory actions as well as in excitatory. An-
tagonisitc reflexes interfere, one reflex deferring, interrupting, or cutting
short another, or precluding the latter altogether fix>m tadi^ing effect
on the final common path. Intraspinal seat of the interference.
Compound reflexes may interfere in part The place ( ? synapse)
where convergent afferent paths impinge on a common path consti-
tutes a mechanism of co-ordination. The convergence of afferent
paths to form common paths occurs with great frequency in the
central nervous system. A question whether any reflexes are in the
intact organism wholly neutral one to another.

We have hitherto dealt with reflex reactions under the guise
of a convenient but artificial abstraction, — the simple reflex.
That is to say, we have fixed our attention on the reaction of a
reflex-arc as if it were that of an isolable and isolated mechanism,
for whose function the presence of other parts of the nervous
system and of other arcs might be negligible and wholly indif-
ferent. This is improbable. The nervous system functions as
a whole. Physiological and histological analysis finds it con-
nected throughout its whole extent. Donaldson opens his

IV] PRINCIPLE OF THE COMMON PATH 115

description of it with the remark: ''A group of nerve-cells dis-
connected from the other nerve-tissues of the body, as muscles
and glands are disconnected from each other, would be without
physiological significance." A reflex reaction, even in a ** spinal
animal " where the solidarity of the nervous system has been so
trenchantly mutilated, is always in fact a reaction conditioned
not by one reflex -arc but by many. A reflex detached from
the general nervous condition is hardly realizable.

The compounding together of reflexes is therefore a main
problem in nervous co-ordination. For this problem it is im-
portant to recognize a feature in the architecture of the gray-
centred (synaptic) nervous system which may be termed " tAe
principle of the common path^^^ If we regard the nervous
system of any higher organism from the broad point of view a
salient feature in its scheme of construction is the following.

At the commencement of every reflex-arc is a receptive
neurone extending from the receptive surface to the central
nervous organ. This neurone forms the sole avenue which
impulses generated at its receptive point can use whithersoever
be their destination. This neurone is therefore a path exclu-
sive to the impulses generated at its own receptive point, and
other receptive points than its own cannot employ it. A single
receptive point may play reflexly upon quite a number of differ-
ent effector organs. It may be connected through its reflex
path with many muscles and glands in many different regions.
Yet all its reflex-arcs spring from the one single shank or
stem, I. ^., from the one afferent neurone which conducts from
the receptive point at the periphery into the central nervous
organ.

But at the termination of every reflex-arc we find a final
neurone, the ultimate conductive link to an effector organ,
(muscle or gland). This last link in the chain, e. g, the motor
neurone, differs obviously in one important respect from the
first link of the chain. It does not subserve exclusively impulses
generated at one single receptive source, but receives impulses
from many receptive sources situate in many and various regions
of the body. It is the sole path which all impulses, no matter

ii6 INTERACTION BETWEEN REFLEXES [Lect.

whence they come, must travel if they are to act on the muscle-
fibres to which it leads.

Therefore, while the receptive neurone forms a private path
exclusively serving impulses of one source only, the final or
efferent neurone is, so to say, a public path, common to impulses
arising at any of many sources of reception. A receptive field,
e. g.^ an area of skin, is analyzable into receptive points. One
and the same effector organ stands in reflex connection not
only with many individual receptive points but even with many
various receptive fields. Reflexes generated in manifold sense-
org^s can pour their influence into one and the same muscle.
Thus a limb-muscle is the terminus ad quem of many reflex-
arcs arising in many various parts of the body. Its motor-
nerve is a path common to all the reflex-arcs which reach that
muscle {cf. infra^ Fig. 44, p. 148).

Reflex-arcs show, therefore, the general features that the
initial neurone of each is a private path exclusively belonging to
a single receptive point (or small group of points) ; and that Anally
the arcs embouch into a path leading to an effector organ ; and
that their flnal path is common to all receptive points whereso-
ever they may lie in the body, so long as they have connection
with the effector organ in question. Before finally converging
upon the motor neurone the arcs converge to some degree.
Their private paths embouch upon inteniuncial paths common
in various degree to groups of private paths. The terminal path
may, to distinguish it from internuncial common paths, be called
the final common path. The motor nerve to a muscle is a
collection of final common paths.

Certain consequences result from this arrangement. One of
these seems the preclusion of essential- qualitative difference
between nerve-impulses arising in different afferent nerves. If
two conductors have a tract in common, there can hardly be
essential qualitative difference between their modes of conduc-
tion ; and the final common paths must be capable of respond-
ing with different rhythms which different conductors impress
upon it. It must be to a certain degree aperiodic. If its dis-
charge be a rhythmic process, as from many considerations it

IV] PRINCIPLE OF THE COMMON PATH 117

appears to be, the frequency of its own rhythm must be capable
of being at least as high as that of the highest frequency of any
of the afferent arcs that play upon it ; and it must be able also
to reproduce the characters of the slowest^

A second consequence is that each receptor being dependent
for final communication with its effector organ upon a path not
exclusively its own but common to it with certain other recep-
tors» such nexus necessitates successive and not simultaneous
use of the common path by various receptors using it to different
or opposed effect. When two receptors are stimulated simultane-
ously, each of the receptors tending to evoke reflex action that
for its end-effect employs the same final common path but em-
plo)rs it in a different way from the other, one reflex appears
without the other. The result is this reflex or that reflex, but
not the two together.*® Excitation of the central end of the
afferent root of the eighth or seventh cervical nerve of the
monkey evokes reflexly in the same individual animal sometimes
flexion at elbow, sometimes extension. If the excitation be
preceded by excitation of the first thoracic root the result is
usually extension : if preceded by excitation of the si-xth cer-
vical root it is usually flexion. Yet though the same root may
thus be made to evoke jeflex contraction of the flexors or of the
extensors, it does not, in my experience, evoke contraction in
both flexors and extensors in the same reflex-response. Of the
two reflexes on extensors and flexors respectively, either the one
or the other results, but not the two together. Thus, in my
experience, excitation of the seventh or eighth root never causes
simultaneously with reflex contraction of the flexors of elbow a
contraction of that part of the triceps which extends the elbow.
The flexor-reflex when it occurs seems therefore to exclude
the extensor-reflex, and vice versa. If there resulted a com-
promise between the two reflexes, so that each reflex had a
share in the resultant, the compound would be an action which
was neither the appropriate flexion nor the appropriate exten-
sion. Were there to occur at the final common path algebraical
summation of the influence exerted on it by two opposed re-
^ BagUoni's results ** support this inference.

ii8 INTERACTION BETWEEN REFLEXES [Lect.

cepdve arcs, there would result in the effector organ an action
adapted to neither and useless for the purposes of either.

In the Coelenterate, Carmarina^ a mechanical stimulus ap-
plied to the subumbrella causes, as in another Geryonid, Tiar-
opsis indicans^ a reflex movement that brings the free end
of the manubrium to the spot touched. Bethe reports ** that
if two stimuli are applied simultaneously to opposite points of
the discoid subumbrella, the points chosen being such that the
manubrium is midway between them, the manubrium is moved
toward the point at which the stimulus applied was the stronger.
He adds that if both stimuli are of exactly equal strength the
manubrium remains unmoved and uncontracted. To obtain
such a result as this last with antagonistic spinal reflexes in the
vertebrate would obviously be more difficult, because the more
complex the preparation and the nervous system involved, the
more difficult it will be at any moment to exactly balance the
two reflexes. But, apart from that, the observation on Carmarina
is an analogue of that in the monkey's ann.

This dilemma between reflexes would seem to be a problem
of frequent recurrence in reflex co-ordination. We note an
orderly sequence of actions in the movement of animals, even
in cases where every observer admits that the co-ordination
is merely reflex. We see one act succeed another without con-
fusion. Yet, tracing this sequence to its external causes, we
recognize that the usual thing in nature is not for one exciting
stimulus to begin immediately after another ceases, but for an
array of environmental agents acting concurrently on the animal
at any moment to exhibit correlative change in regard to it, so
that one or other group of them becomes — generally by in-
crease in intensity — temporarily prepotent Thus there domi-
nates now this group, now that group in turn. It may happen
that one stimulus ceases coincidently as another begins, but as a
rule, one stimulus overlaps zxio'^^x in regard to time. Thus each
reflex breaks in upon a condition of relative equilibrium, which
latter is itself reflex. In the simultaneous correlation of reflexes
some reflexes combine harmoniously, being reactions that mu-
tually reinforce. These may be termed allied reflexes^ and

IV]

PRINCIPLE OF THE COMMON PATH

119

the neural arcs which they employ allied arcs. On the other
hand, some reflexes, as mentioned above, are antagonistic one to
another and incompatible. These do not mutually reinforce,
but stand to each other in inhibitory relation. One of them
inhibits the other, or a whole group of others. These reflexes
may in regard to one another be termed antagonistic ; and the
reflex or group of reflexes which succeeds in inhibiting its
opponents may be termed " prepotent " for the time being.

Fksurk 38. — Summation effect {immediate spinal induction) between the arcs Ra and R/lof
Fig. 39 B. PC the flexor muscle of the hip. Sa the signal line marking the period of
stimulation of the skin belonging to arc Ra (Fig. 39 B) of the shoulder skin. The strength
of stimulus is arranged to be subminimal, so that a reflex-response in fc is not obtained.
Stf the signal line marking the period of stimulation, also subminimal of a point of shoulder
skin 8 centimeters from Ra. Though the two stimuli applied separately are each unable to
evoke the reflex, when applied contemporaneously they quickly evoke the reflex. The
two arcs R« and R^, therefore, reinforce each other in their action on the final common
path PC. Time in fifths of seconds. Read from left to right

120 INTERACTION BETWEEN REFLEXES [Lect.

Allied reflejwB. The action of the principle of the final com-
mon path may be instanced in regard to " allied arcs " in the
scratch-reflex as follows. If, while the scratch-reflex is being
elicited from a skin point at the shoulder, a second point
distant, e. g. lO cent from the other point but also in the re-
ceptive fleld of skin, be stimulated, the stimulation at this
second point favours the reaction from the flrst point. This is
well seen when the stimulus at each point is of subminimal in-
tensity. The two stimuli, though each unable separately to
invoke the reflex, yet do so when applied both at the same time
(Fig. 38). This IS not due to overlapping spread of the feeble
currents about the stigmatic poles of the two circuits used.
Weak cocainization of either of the two skin points annuls it
Moreover, it occurs when localized mechanical stimuli are used.
It therefore seems that the arcs from the two points, e. g. Ra
and R/9 (Fig. 39 B) have such a mutual relation that reaction
of one of them reinforces reaction of the other, as judged by the
effect on the flnal common path.

It is obvious that such reinforcement — immediate spinal
induction may occur in either of two ways. The diagram
(Fig- 39 B) treats the final common path as if it consisted of
a single individual neurone. The single neurone of the dia-
gram stands for several thousands. It may be (i) that when
the reflex is excited from Ra only a particular group of the
motor neurones composing the final common path is thrown
into action, and similarly another particular group when the
reflex is excited from R/8. If the two groups in the final
common path are separate g^roups, the explanation of the
reinforcement shown in the muscular response may be by me-
chanical summation of contraction occurring in two separate
fields of muscular tissue, the contraction of each too slight to
cause perceptible movement by itself without the other. In
other words, the reinforcement would be due not to the response
in the set of neurones comprising the final common path (fc
Fig- 39 B)» being neurone for neurone more intense under the
combined stimulation of Ra and R)3 than under stimulation
of either singly, but the result would arise from the number of

IV]

ALLIED REFLEXES

121

FiGURB 39. — A. The " recq^tire fidd,*' as rerealed after low cenrical transection, a saddle-
shaped area of dorsal skin, whence the scratch-reflex of the left hind limb can be evoked.
Ir marks the position of the last rib.

B. Diagram of the spinal arcs involved, l, receptire or afferent nerve-path from
the left foot ; r, receptive nerve-path from the opposite foot ; r«, R^, receptive nerve-
paths from hairs in the dorsal sldn of the left side; fc, the final common path, in this
case the motor neurone to a flexor muscle of the hip ; p«, p^, proprio-spinal neurones.

neurones in action in FC being simply greater under the stimu-
lation of the two skin points than under stimulation of one of
them only.

On the other hand, it may be (2) that all the neurones com-
posing the final common path constitute together one almost
unitary apparatus, so that stimulation at Ra excites or can

122 INTERACTION BETWEEN REFLEXES [Lect.

excite them all, and similarly stimulation at R/3 excites or
can excite them all. The question, therefore, regarding the
mode of the reinforcement is a question between intensity and
extensity. The scratch-reflex affords some opportunity for ex-
amining this question. The rhythm of the reflex has practically
the same frequency whether the reflex be excited strongly or
feebly : thus, whether the amplitude of the contractions be great
or small, they recur with practically the same frequence. Sup-
pose the reflex be excited by stimulation of the skin point R a
(I^ig- 39 B), and suppose the stimulus is weak, producing only a
feeble reflex. Then let another skin point Rfi (Fig. 39 B) be
stimulated while Ra is being stimulated, and let the stimuli at
K/3 be timed so as to fall alternately with those applied at Ro.
Then if the two paths impinge on two diflierent sets of units in
the compound group of motor neurones composing the flnal
common path, evidence of two rhythms should appear, for the
muscle-flbres (of the flexors of the hip) can respond to a much
quicker rhythm than four per second. But, in fact, the result is
that the rhythm appears unquickened and unaltered (Figs. 18,
19, 20, 21). There is not even a break or interference in it
It might be thought, therefore, that for some reason the stimu-
lation of the second point, Rfi, is remaining ineffective alto-
gether. But that is not so, because the stimulation at R)3.has
often the effect of increasing the amplitude (Fig. 18) of the
individual beats of the rhythmic reflex, though it does not alter
the rhythm. This change in amplitude proves that the reflex is
also in action from the second skin point as well as from the flrst.
But there is no interference of the rhythms of the two reflexes.
Evidently the central mechanism on which R/3 acts is subjected
by Ra to a refractory state which the stimulation at Rff does
not break through. That is, the refractory state obtaining in
the central mechanism under action from Ra obtains at the
same moment for excitation reaching it from R/8. The central
mechanism acted on by Rff must therefore belong in common
to the reflexes from Ra and Rfi respectively. And since the
experiment can be repeated with a great number of different
pairs of points in the receptive field, practically the whole of the

W] IMMEDIATE SPINAL INDUCTION 123

neurones of FC are common to all the receptive points in the
receptive field. Similarly it is shown by Zwaardemaker '^ that
the refractory phase demonstrated by him in reflex deglutition
spreads to the whole of the reflex centre, both right and left

Again, it was shown above, under the heading of summation,
that although a single-induction shock, even though strong, does
not in my experience ever evoke a scratch-reflex, a series of
even feeble shocks does so by summation. But in order to act
by summation the individual shocks must follow each other at
not too long an interval of time, the interval being caeteris pari-
ims shorter the less intense the shocks. Suppose an induction
shock be applied to Ra at such a frequence, e, g. once a second,
that at the intensity chosen they fail to evoke the reflex. Sup-
pose that a series of induction shocks be applied to R)3 similarly
unable to evoke the reflex. Then suppose that while the
stimuli are being applied to Ra and fail to evoke the reflex,
the other series of stimuli are applied to R/S, and are so ap-
plied that each stimulus at R/S falls at a moment of time mid-
way between the moments of application of the stimuli at Ra.
The stimuli thus conjoined suflice to evoke the reflex. Evi-
dently the internal excitatory change is not confined to the arcs
to whose receptive ends the external stimulus is actually applied.
It spreads to other arcs belonging to the same " type-reflex,"
especially to those arising near to those actually stimulated in
the receptive field. A subminimal stimulus at one point in the
field favours response to a subsequent stimulus at a second point
in the field even 8 centimeters distant — so long as the second
stimulus follows within summation time; but the summation
time is shorter than when stimuli follow each other at one and
the same spot, and is especially so when the points stimulated
lie distant from one another. Hence we may draw some sort
of picture of the extent of the excitatory internal change in-
duced in this reflex mechanism by a single momentary stimu-
lus: as to distribution in time the change fades off gradually
from an early maximum to a trace just detectable after 1400 cr,
if the stimulus be strong : as to distribution in space it spreads
from the peripherally stimulated arcs A A themselves as centre

124 INTERACTION BETWEEN REFLEXES [Lect.

to the intraspinal parts of other arcs of the same type-reflex, but
among these it affects those starting in the skin as neighbours
to A A more than ones more distant in origin, and it endures
less long in these than in its own arcs ; hence the shorter sum-
mation time. Exner early insisted on the close connection
between " facilitation " (bahnung) and summation. The aboive
immediate spinal induction illustrates it welL

The mutual reinforcement of action exercised by the two
scratch-reflexes one upon another appears therefore to be an
affair of intensity. This does not» however, exclude the exist-
ence of extensity as a factor also in some degree. There is
evidence, adverted to above (p. 76, Lect III), that makes it likely
that in very weak reflexes not all the individual neurones
composing the final common path are in action, although in
stronger reflex reactions all may be in action.

In the scratch-reflex the mutual reinforcing power between
the reflexes falls as the distance between the receptors of the
arcs increases. The nearer the skin points of Ra and K/3 lie
together the greater the mutual reinforcement between the action
of their arcs on FC. This suggests an explanation by physical
diffusion of the stimulating currents applied to Ra and Rfi; but
for the reasons above mentioned this overlap of stimulus can, I
consider, be excluded. Light is however thrown on this propor-
tion between the degree of reinforcement and the degree of
nearness of the receptive points by another feature of the re-
flex. The scratch-reflex in the spinal dog carries the foot ap-
proximately toward the place of stimulation. In the spinal dog
the reflex does not succeed in bringing the foot actually to the
irritated skin point, yet when the irritation lies far forward
the foot is carried further forward, and when the irritation is
far back the foot is carried further back. A scratch-reflex
evoked by a stimulus applied far back and high up in the
dorsal skin is therefore not wholly like a scratch-reflex evoked
from far forward and low down. These differences are easily
registered in graphic tracings of the movement at hip (Fig.
40). It is found that the greater the likeness between the two
scratch-reflexes which two separate skin points initiate, the

INTERACnON BETWEEN REFLEXES [Lect.

siraiign' the mtitual retaforcesaeot between the action of those
two receptive poiiits upon the final coamioQ path FC (Flfs. 38 and
41). In other words, the coalition beti»-cen reflexes is greater the
greater the likeness between them, and that likeness increases
with the nearness of th^ receptive points to one another in
the skin surface. I have seen the mutual reinforcement demon-
strable with skin points 20 centimeters apart in the receptive
field of the scratch- redex, but I have failed to find this mutual
reinforcement between the most distant arcs of the receptive
field. Whether coalition fades into mere indiflference or passes
over into antagonism I have not at present the evidence to
judge.

The whole collection of points of skin surface bom which
the scratch-reflex can be elicited may conveniently be termed
the rueptivt fit'hi of that reflex. And the receptive field of a
reflex is analyzable into points from each of which the reflex
can be evoked. But the reflex as elicited from various points

IV] TYPE-REFLEXES AND ALLIED REFLEXES 127

in its receptive field is not in the case of all the points exactly
the same reflex ; e. g.^ the foot is directed to somewhat diflferent
places according as the scratch-reflex is elicited from this or that
point. A similar feature is seen in the ** wisch-reflex " of the
spinal frog*s hind leg. That is to say, when we speak of the
" scratch-reflex " in general, what we mean strictly speaking is a
group of reflexes all more or less alike, all using approximately the
same motor apparatus in approximately the same way, and all
more or less conforming to the same type. And this group of indi-
vidual reflexes forms a physiological group not only on account
of their similarity, but also because they act harmoniously upon
the same final common path, and in many cases reinforcement
occurs between them in their action on that common path.
Their intraspinal mechanisms are more or less knit together
into an harmonious whole. A reflex, e. g. the scratch-reflex,
when referred to in general, may be conveniently termed a
type-reficx. The kind of harmonious relationship which holds
between the individual reflexes comprised under one and the
same type-reflex may be indicated by recognizing them as ** al-
lied reflexes*' and their arcs as *' allied arcs."

Similarly with the various other reflexes. The flexion-reflex
of the hind limb, the pinna-reflex, the extensor-thrust, the
crossed extension-reflex of the hind limb, the torticollis reflex,
etc, ; these are each of them type-reflexes. Each is a group of
reflexes. The individual reflexes comprised in each of these
type-reflexes have such mutual relationship between themselves
that they act harmoniously together on the same final common
path, and are therefore ** allied reflexes*' and employ ''allied
arcs."

The extent of the receptive field of each type-reflex is usually
wide. It IS much wider in some type-reflexes than in others;
thus, that of the direct flexion-reflex of the hind limb of the
dog is more extensive than that of the extensor-thrust of the
limb. Within the receptive field of any given type-reflex not all
the receptive points equally potently excite the reflex. From
certain areas of points the reflex can be most easily evoked, from
certain others least easily, and from the rest of the field with in-

128 INTERACTION BETWEEN REFLEXES [Lect.

termediate degrees of facility. The area whence the reflex can
be evoked with most difficulty is usually the circumferential zone
of the field, the width of the zone varying along different radii.
The area where the threshold stimulus is lowest lies usually £urly
remote, though not equally remote, firom all the borders of the
field. The reflex effect of a weak stimulus in this central focal
area seems to resemble the effect of a stronger stimulus applied
in the border zone of the field. Reflexes of an intensity unob-
tainable from the border zone of the field can be easily provoked
by stimulation of the focal area of the field. In the flexion-reflex
of the dog's hind limb the toe-pads and plantar cushion are in
the focal area of the receptive field. In the scratch-reflex of the
dog the focal area is along that part of the field that lies next to
the mid-dorsal line of the trunk, and especially (as seen after
low cervical transection) near the posterior end of the scapular
region; e.g, in Fig. 39 A, from 5 to 15 in the horizontal figures
^ and dorsal to 9 in the vertical row. The difference between the
threshold value of the stimulus for the reflex at different points
in the field is very considerable indeed. Although the absolute
value of the threshold may vary considerably in one and the
same animal at different times, even from day to day, the rela-
tive values as between separate areas in the same field is
usually about the same. But this relative value may be upset by
** local fatigue," etc. The coalescence of allied reflexes excited
from one receptive field tends to make weak stimuli applied to an
extensive area equivalent to intenser stimuli applied to a smaller
area. G. H. Parker*" shows that in the positive phototropism of
the frog to light falling on its skin the strength of the reaction
varies in proportion with the extent of skin exposed to the light.
Reflex complication. One and the same field of receptive
surface may, and usually does, contain receptive points of more
than a single species. Thus, a skin-field may contain receptors
some of which are adapted for mechanical stimuli, some for
chemical, some for thermal, and so on. In this case receptors
of two different species may not both of them initiate reflexes
which belong to the same type-reflex, 1. e. which have the rela-
tion to one another of " allied reflexes." For instance, in the

rV] RECEPTIVE FIELD OF A REFLEX 129

planta of the dog's foot receptors coexist *" of which one set
are excited by mechanical stimuli of harmless ^tactual) kind, the
other set by stimuli of nocuous kind. The reflexes elicited from
the limb through these two kinds of receptors respectively
do not reinforce each other but oppose each other. On the
other hand, in the tentacles of the Actinian, Aiptasis saxicola,
there coexist at the surface receptors of two species,**® one re-
ceptive for tactual stimuli the other for certain chemical stimuli
(Nagel). The reflexes elicited through these by combination
of mechanical with certain chemical stimuli seem to combine
harmoniously and mutually reinforce each other (Nagel). And
a similar occurrence seems evidenced by observations on the
barblets of Siluroid fishes, e.g, Ameiurus*^ (C J. Herrick).
The combining of such reflexes is comparable with the associa-
tive combination of disparate sensations for which Herbart**
introduced the term "complication."

Analogy exists here, as it should, between the compatibility
of reflex movements from two receptors of different species and
the compatibility of sensations which, judging by inference from
our own introspection, might be initiated from such receptors.
Skin-pain is sensually incompatible with pure touch, the dolor-
ous suppressing the tactual, just as the noci-ceptive reflex in the
" spinal " dog's hind leg suppresses the merely tango-ceptive.
But gustatory and tactual sensations excited from the same re-
ceptive surface, e, g, the tongue, habitually blend harmoniously.

Proprio-ceptive reflexes. There exists a further important class
of cases in which reflexes have " allied " relation. Throughout
a vast range of animal types the bulk formed by the organism
presents to the environment a surface sheet of cells, and, beneath
that, a mass of cells more or less screened from the environment
by the surface sheet Many of the agencies by which the envi-
ronment acts on the organism do not penetrate it far enough to
reach the cells of the deep mass inside. Bedded in the surface
layer of the organism are numbers of receptor cells constituted
in adaptation to the stimuli delivered by environmental agencies.
But the organism itself, like the world surrounding it, is a field
of ceaseless change, where internal energy is continually being

I30 INTERACTION BETWEEN REFLEXES [Lect.

liberated, whence chemical, thermal, mechanical, and electrical
effects appear. It is a microcosm in which forces which can act
as stimuli are at work as in the macrocosm around. The deep
tissues underlying the surface sheet are not provided with
receptors of the same kinds as those of the surface, yet they
are not devoid of receptors. They have receptors specific to
themselves. The receptors which lie in the depth of the organism
are adapted for excitation consonantly with changes going on in
the organism itself, particularly in its muscles and their acces-
sory organs (tendons, joints, blood-vessels, etc). Since in this
field the stimuli to the receptors are given by the organism itself,
their field may be called the proprio-ceptive field.

There exist, therefore, two primary distributions of the re-
ceptor organs, each a field in certain respects fundamentally
different from the other. The surface field lies freely open to
the numberless vicissitudes of the environment. It has felt for
countless ages the full stream of the varied agencies forever
pouring upon it from the outside world. This field, exUnhcepttve
as it may be called, is rich in the number and variety of re-
ceptors which adaptation has evolved in it

The excitation of the receptors of the propruhceptive field
in contradistinction from those of the exterv-ceptive is related
only secondarily to the agencies of the environment. The
proprio-ceptive receive their stimulation by some action, e, g.
a muscular contraction, which was itself a primary reaction to
excitation of a surface receptor by the environment. The pri-
mary reaction is excited in the majority of cases by a receptor
of the extero-ceptive field, that field so rich in the number and
the variety of its receptors. Reflexes arising from proprio-
ceptive organs come therefore to be habitually attached and
appended to certain reflexes excited by extero-ceptive organs.
The reaction of the animal to stimulation of one of its extero-
ceptors excites certain tissues, and the activity thus produced in
these latter tissues excites in them their receptors, which are
proprio-ceptors. Thus, in a muscular movement induced by a
stimulus to the skin of the spinal dog, the change in form and
tension of the muscles, the movements of the joints, etc., excite

IV] PROPRIOCEPTIVE AND EXTERO-CEPTIVE 131

the receptors in these structures, and these in turn initiate a
reflex in their own arcs and their reaction often has an " allied "
relation to the reflex reaction excited from the skin.

Alliance of proprio-ceptive with eztero-oeptive reflexes. In
one of the type-reflexes previously described, namely the scratch-
reflex, the reflex-arcs which provoke the reflex arise in a
large continuous area of skin, and all excite the same motor
neurones, that is, are mutually related as allied arcs. The
area of skin whence these arcs arise we termed the recep-
tive field of the reflex. The afferent nerves of the muscles
which execute the scratching movement do not, when them-
selves excited, evoke the scratch-reflex; nor does the sever-
ance of the afferent nerves of the muscles obviously impair
or alter the scratch-reflex. With the flexion-reflex of the limb
it is different. The reflex, like the scratch-reflex, has a cutane-
ous field of origin. It is provocable from arcs arising in a
large area of the skin covering the hind limb. But the flexion-
reflex can in addition be excited from various of the affierent
nerves of the muscles of the limb. Thus stimulation of the
central end of the nerve of the flexor muscles themselves excites
the reflex. It is similarly elicitable from the afferent nerve of
the extensor muscle (vasto crureus) of the knee. And the
reflex excited from the muscles of the limb allies itself with
the reflex excited from the skin of the limb. A subliminal
stimulation of the aff*erent nerve of the hamstring muscles
applied simultaneously with a subliminal stimulation of the
skin of the foot results in a marked flexion-reflex.

In the case of the flexion-reflex, therefore, the receptive field
includes not only reflex-arcs arising in the surface field, but
reflex-arcs arising in the depth of the limb. Combined there-
fore with an exteroceptive area, this reflex has, included in its
receptive field, ^proprioceptive field. The reflex-arcs belonging
to its extero-ceptive and proprio-ceptive components co-operate
harmoniously together, and mutually reinforce each other's
action. In this class of cases the reflex from the muscle-
joint apparatus seems to reinforce the reflex initiated from the
skin.

132 INTERACTION BETWEEN REFLEXES [Lect.

Reflex flexion of the leg is induced by stimulation of the
central end of the nerve of a hamstring muscle. Since me-
chanical stimulation of these flexor muscles, e. g. kneading or
squeezing them, excites a reflex inhibition of the contraction of
their antagonists, which as we have seen is part of the flexion-
reflex itself, it would seem likely that their own contraction will
excite a flexion-reflex. A flexion-reflex excited from the skin
would thus in its progress tend to induce a secondary flexion-
reflex which would reinforce the primary one, for when excited
apart the reflexes excited from an afferent nerve of the foot and
from the hamstring nerve are closely similar (Fig. 37). The
case therefore resembles that of the reflexes from two adjacent
spots in the receptive field of the scratch-reflex. The reflex
elicited from the skin of the foot and that elicited from the
hamstring muscle are " allied " reflexes. There is here alliance
and "bahnung" between a reflex of the proprio-ceptive field and
a reflex of the extero-ceptive field.

Similarly, if the knee-jerk is accepted as a sign of a tonic
reflex originated by the afferent nerve-endings in the knee-jerk
muscles themselves, many reflexes elicitable from the extero-
ceptive surface are well known to reinforce it A comprehensive
account of these was furnished in Sternberg's monograph ^^
(1893). Here again the reflexes which are "allied," exhibiting
reinforcement and "bahnung," belong not in the ordinary sense
to the same categor>% but have reflex-arcs commencing in
receptive organs of different species. Yet the arcs are " allied
arcs," for they act harmoniously on the same final common path.

That the prolongation of the reflex contractions character-
istic of strychnine is due to excitation of muscular (proprio-
ceptive) reflexes (Baglioni) ^'' ^^ secondary to a reflex elicited
from other receptors is again a further illustration of the
secondary relation of proprio-ceptive reflexes to extero-ceptive
pointed out above.

"Wider combinations of reflexes. And reflexes whose arcs
commence in receptive fields even wider apart than those men-
tioned above may also have " allied " relation. In the bulbo-
spinal dog stimulation of the outer digit of the hind foot will

IV] COMBINATIONS OF ALLIED REFLEXES 133

evoke reflex flexion of the leg, and stimulation of each of the
other digits evokes practically the same reflex ; and if stimula-
tion of several of these points be simultaneously combined the
same reflex as a result is obtained more readily than if one
only of these points is stimulated. And to these stimulations
may be added simultaneously stimulation of points in the
crossed fore foot; stimulation there yields by itself flexion ot
the hind leg; and under the simultaneous stimulation of fore
and hind foot the flexion of the leg goes on as before, though
perhaps more readily; that is, the several individual reflexes
harmonize in their effect on the hind limb. Further, to these
may be added simultaneous stimulation of the tail, and of the
crossed pinna ; and the reflexes of these stimulations all coa-
lesce in the same way in flexion of the hind leg. Exner*
has shown that in exciting diff*erent points of the central ner-
vous system itself, points widely apart exert bahnung for one
another's reactions, and for various reflex reactions induced
from the skin. Thus reflexes originated at different distant
points, and passing through paths widely separate in the brain,
converge to the same motor mechanism (final common path)
and act harmoniously upon it. Reflex-arcs from widely differ-
ent parts conjoin and pour their influence harmoniously into
the same muscle. The motor neurones of a muscle of the
knee are the tenntnus ad quern of reflex-arcs arising in re-
ceptors not only of its own foot, but from the crossed fore
foot and pinna, and tail, also undoubtedly from the otic laby-
rinth, olfactory organs, and eyes. Thus, if we take as a stand-
point any motor-nerve to a muscle it consists of a number of
motor neurones which are more or less bound into a unit
mechanism ; among the reflex-actions of the organism a number
can all be brought together as a groups because they all in
their course converge together upon this motor mechanism,
this final common path, activate it, and are in harmonious
mutual relation with regard to it. They are in regard to it
what were termed above " allied " reflexes.

Allied inhibitory reflexes. The examples of allied reflexes
cited so far hav^ had for their result on the final common path

134 INTERACTION BETWEEN REFLEXES [Lect.

an increase of its activity ; that is to say, of its activity as a dis-
charger of nervous impulses. But the same final common path
can be shown to be connected also with certain reflexes initi-
able fi-om other receptive points which depress its activity as
a discharger of ner/ous impulses. The reflexes exerting this
influence are '' inhibitory," whereas the reflexes mentioned be-
fore may be termed " excitatory." Inhibitory reflexes are
accessible to study chiefly through the kind of refi-actory state
which they impress upon the commencement of the efferent
part of their arc, as tested by concurrent excitations of reflexes
which should excite it.

Just as in regard to one and the same final common path
certain excitatory reflexes act harmoniously together and rein-
force one another, so also do certain inhibitory reflexes. Thus,
reflex inhibition of the flexors of the knee (spinal dog) is
regularly excitable by stimulation of the skin of a digit of the
crossed hind foot; and the concurrent stimulation of two or
more digits and of the dorsum pedis of the crossed foot mutu-
ally combine and reinforce in their reflex inhibition of the
knee-flexor: and to these may be added stimulation of the
homonymous fore foot : all these reflexes combine harmoniously
together in exerting a conjoint inhibitory influence on the knee-
flexors. The alliance between reflexes in regard to any one
final common path may be as wide and strong when the end-
result of those reflexes is in the form of inhibition as when it is
in the form of excitation. In addition, therefore, to the category
of "allied excitatory" reflexes above mentioned there is a
category of " allied inhibitory " reflexes. Under this latter
category come subgroups analogous to the four already men-
tioned under allied excitatory reflexes. Thus : the reflex from
the proprio-ceptive nerves of the hamstring muscles combines
with and reinforces the flexion-reflex from the skin of the foot
of the same leg in a resultant reflex inhibition of the extensors
of the homonymous knee.

But there are, as we have seen, reflexes which are neither
purely excitatory nor purely inhibitory. For instance, the
flexion-reflex of the hind leg (cat and dog) is, as we have seen.

IV] ANTAGONISTIC REFLEXES 135

at one and the same time excitatory of the flexor neurones
(knee) and inhibitory of the extensor neurones (knee).

These reflexes of simultaneous double-sign may have " allied "
relation with one another, e. g.^ the individual reflexes of the
flexion type-reflex.

Also there are other reflexes neither purely excitatory nor
purely inhibitory, namely, the reflexes which during the con-
tinuance or repetition of the exciting stimulus exhibit refractory
period. Several rhythmic reflexes seem of this character, e. g.
the swallowing reflex, the scratch-reflex. If we regard refrac-
tory phase as a kind of inhibition, then these reflexes are, as
we have seen, reflexes of successive double-sign. And these
also can be " allied" in their relation one to another.

Antagonistio reflexes. But not all reflexes connected to one
and the same final common path stand to one another in the
relation of " allied reflexes." Suppose during the scratch-reflex
a stimulus be applied to the foot not of the scratching side but
of the opposite side (Fig. 39 B, r). The left leg, which is ex-
ecuting the scratch-reflex in response to stimulation of the Uft
shoulder skin is cut short in its movement by the stimulation of
the right foot, although the stimulus at the shoulder to provoke
the scratch movement is maintained unaltered all the time. The
stimulus to the right foot will temporarily interrupt a scratch-
reflex, or will cut it short or will delay its onset ; which it does
of these depends on the time-relations of the stimuli (Fig. 42).
The inhibition of the scratch-reflex occurs sometimes when the
contraction of the muscles innervated by the reflex conflicting
with it is very slight. There i3 interference between the two
reflexes and the one is inhibited by the other. The final com-
mon path used by the left scratch-reflex is also common to the
reflex elicitable from the right foot This latter reflex evokes
at the opposite (left) knee extension; in doing this it causes
steady excitation of extensor neurones of that knee and steadily
inhibits the flexor neurones.** But the scratch-reflex causes
rhythmic excitation of the flexor neurones. Therefore these
flexor neurones in this conflict lie as a final common path under

136 INTERACTION BETWEEN REFLEXES [Lect.

the influence of two antagonistic reflexes, one of which would
excite them to rhythmical discharge four times a second, while
the other would continuously repress all discharge in them.
There is here an antagonistic relation between reflexes em-
bouching on one' and the same flnal common path.

In all these forms of interference there is a competition, as
it were, between the excitatory stimulus used for the one reflex
and the excitatory stimulus for the other. Both stimuli are in
progress together, and the one in taking effect precludes the
other's taking effect as far as the flnal common path is con-
cerned; and the precise form in which that occurs depends
greatly on the time-relations of application of the two stimuli
competing against each other.

Again, if, while stimulation of the skin of the shoulder is
evoking the scratch-reflex, the skin of the hind foot of the same
side is stimulated, the scratching may be arrested**^ (F>g- 43)-
Stimulation of the skin of the hind foot by any of various
stimuli that have the character of threatening the part with
damage causes the leg to be flexed, drawing the foot up by
steady maintained contraction of the flexors of the ankle, knee,
and hip. In this reaction the reflex-arc is (under schematic
provisions similar to those mentioned in regard to the scratch-
reflex schema) (!) the receptive neurone (Fig. 39 B, L). noci-cep-
tive, from the foot to the spinal segment, (ii) the motor neurone
(F>g- 39 B, FC) to the flexor muscle, e.g. of hip (a short intra-
spinal neurone; a Schalt-zelle (v. Monakow) is probably ex-
istent between (i) and (ii) but omitted for simplicity). Here,
therefore, there is an arc which embouches into the same final
common path FC as do Ra and R^, Fig. 39 B. The motor
neurone FC is a path common to it and to the scratch-reflex

Figure 42 (opposite). — Interference of the reflex from the skin of the opposite foot with the
scratch-reflex, fc, the flexor muscle of the left hip (Fig. 39 B, fc). r, the signal line the
notch in which marks the beginning, continuance, and conclusion of a skin stimulation of
the right foot (Fig. 39 B, r.) s, signal line similarly marking the period of stimulation of
the skin of the left shoulder (Fig. 39 B, Ro). The ability of stimulus s to produce the
scratch-reflex takes effect only on concluding stimulus r ; that is, s obtains connection with
theyfna/ common path (the motor neurone of the flexor muscle) only on r's relinquishing
it. Stimulus r, while excluding s from fc, causes slight contraction of fc*s antagonist,
and coincident slight relaxation of fc itself. Time in fifths of seconds. Read from left
to right.

IV]

ANTAGONISTIC REFLEXES

137

138 INTERACTION BETWEEN REFLEXES [Lect.

arc ; both these arcs employ the same effector organ, namely,
the knee-flexor, and employ it by the common medium of
the final path FC. But though the channels for both reflexes
embouch upon the -same final common path, the excitatory
flexor effect specific to each differs strikingly in the two cases.
In the scratch-reflex the flexor effect is an intermittent effect;
in the noci-ceptive flexion-reflex the flexor effect is steady and
maintained. The accompanying tracing (Fig. 43) shows the
result of conflict between the two reflexes. The one reflex
displaces the other at the common path. Compromise is not
evident The scratch-reflex is set aside by that of the noci-cep-
tive arc from the homonymous foot. The stimulation which
previously sufficed to provoke the scratch-reflex is no longer
effective, though it is continued all the time. But when the
stimulation of the foot is discontinued the scratch-reflex returns.
In that respect, although there is no enforced inactivity there is
an interference which is tantamount to, if not the same thing as,
inhibition. Though there is no cessation of activity in the
motor neurone, one form of activity that was being impressed
upon it is cut short and another takes its place. A stimulation
of the foot too weak to cause more than a minimal reflex will
often suffice to completely interrupt, or cut short, or prevent
onset of, the scratch-reflex.

The kernel of the interference between the homonymous
flexion-reflex and the scratch-reflex is that both employ the
same final common path FC to different effect — just as in
the interference between the crossed extension-reflex and the
scratch-reflex. Evidently, the homonymous flexion-reflex and
the crossed extension-reflex both use the same final common
path FC. And they use it to different effect. The motor neurone

Figure 43 (opposite). — Interference between the reflex action of the left hip flexor, pc, caused
by the nervous arc from the left foot (l, Fig. 39 B) and the scratch-reflex. The stimula-
tion of the dorsal skin (Fig. 39 A) inducing the scratch-reflex began at the beginning of the
notch in the signal line s, and continued throughout the period of that notch. Later, for the
period marked by the notch in signal line l, the stimulation of the foot was made. This
latter stimulation interrupts the clonic scratch-reflex in the manner shown. The time is
registered above in fifths of seconds. The tracing reads from left to right. It is note-
worthy that the interruption of the scratch-reflex by the foot-reflex is not established
directly the foot-stimulus begins, and that it outlasts for a short time the applicatioD of
the foot-stimulus.

rv]

ANTAGONISTIC REFLEXES

139

I40 INTERACTION BETWEEN REFLEXES [Lect.

to the flexor of the knee being takeii as representative of the
final common path, the homonymous flexion-reflex excites it
to discharging activity, but the crossed extension-reflex inhibits
it from discharging. Hence if, while the direct flexion-reflex is
in progress the crossed foot is stimulated, the reflex of the knee-
flexor is inhibited. The crossed extension-reflex therefore in-
hibits not only the scratch-reflex but also the homonymous
flexion-reflex.

Further, in all these interferences between reflexes the direc-
tion taken by the inhibition is reversible. Thus, the scratch-
reflex is not only liable to be inhibited by, but is itself able
to inhibit, either the homonymous flexion-reflex or the crossed
extension-reflex; the homonymous flexion-reflex is not only
capable of being inhibited by the crossed extension-reflex (Fig.
32, p. 98), but conversely in its turn can inhibit the crossed
extension-reflex (Figs. 33, 35, p. 100). These interferences are
therefore reversible in direction. Certain conditions determine
which reflex among two or more competing ones shall obtain
mastery over the final common path and thus obtain expression.

Therefore, in regard to the final common path FC the
reflexes that express themselves in it can be grouped into sets,
namely those which excite it in one way, those which excite
it in another way, and those which inhibit it The reflexes
composing each of these sets stand in such relation to reflexes
of the same set that they are with them " allied reflexes." But
a reflex belonging to any one of these sets stands in such
relation to a reflex belonging to one of the other sets that it
is in regarfl to the latter an ** antagonistic " reflex. This cor-
relation of reflexes about the flexor neurone in the leg so that
some reflexes are mutually allied and some are mutually antag-
onistic in regard to that neurone, may serve as a paradigm of
the correlation of reflexes about every final common path, e, g,
about every motor nerve to skeletal muscle.*^

As to the intimate nature of the mechanism which thus, by
summation or by interference, gives co-ordination where neurones
converge upon a common path it is difficult to surmise. In the
central nervous system of vertebrates, afferent neurones A and

IV] SUMMATION AND INTERFERENCE 141

B in their convergence toward and impingement upon another
neurone Z, towards which they conduct, do not make any
lateral connection directly one with the other — at least there
seems no clear evidence that they do. It seems then that the
only structural link between A and B is neurone Z itself. Z
itself should therefore be the field of coalition of A and B if
they transmit "allied" reflexes.

It was argued above (Lecture III), from the morphology of
the perikaryon, that it must form, in numerous cases, a nodal
point in the conductive lines provided by the neurone. The
work of Ramon-y-Cajal, van Gehuchten, v. Lenhoss^k, and
others with the methods of Golgi and Ehrlich, establishes as
a concept of the neurone in general that it is a conductive unit
wherein a number of branches (dendrites) converge toward, meet
at, and coalesce in a single outgoing stem (axone). Through this
tree-shaped structure the nervous impulses flow, like the water
in a tree, from roots to stem. The conduction does not normally
run in the reverse direction. The place of junction of the den-
drites with one another and with the axone is commonly the
perikaryon. This last is therefore a nodal point in the conduc-
tive system. But it is a nodal point of particular quality. It is
not a nodal point where lines meet to cross one another, nor
one where one line splits into many. It is a nodal point where
conductive lines run together into one which is the continuation
of them all. It is a reduction point in the system of lines. The
perikaryon with its convergent dendrites is therefore just such
a structure as spatial summation and immediate induction would
demand. The neurone Z may well, therefore, be the field of
coalition, and the organ where the summational and inductive
processes occur. And the morphology of the neurone as a
whole is seen to be just such as we should expect, arguing from
the principle of the common path.

With the phenomenon of " interference " the question is more
difficult There it is not clear that the field of antagonism is
within the neurone Z itself. The field may be synaptic. We
have the demonstration by Verworn^^ that the interference pro-
duced by A at Z for impulses from B is not accompanied by

142 INTERACTION BETWEEN REFLEXES [Lect.

any obvious change in excitability of the axone of Z. Z, if itself
the seat of inhibition, might have been expected to exhibit that
inhibition throughout its extent. This, as tested by its axone,
it does not do. There exist, it is true, older experiments by
Uspensky,** Belmondo and Oddi,^^ etc., according to which
the threshold of direct excitability of the motor root is lowered
by stimulation of the afferent root. This points to an exten-
sion of the facilitation effect through the whole motor neurone,
conversely to Verwom's demonstration for central inhibition.
Verworn's experiment and its result is very clear. It leads us
to search for some other mechanism common to A and B to
which might be attributable their mutual influence on each
other's reactions. But if we admit the conception, argued
above (Lecture I), that at the nexus between A and Z, /• e. at
synapse A Z, and similarly between B and Z, /. e. at synapse
B Z, there exists a surface of separation, a membrane in the
physical sense, a further consequence seems inferable. Suppose
a number of different neurones A, B, C, etc., each conducting
through its own synapse upon a neurone Z. The synapses
A Z, B Z, C Z, etc., are all surfaces or membranes into which Z
enters as a factor common to them all. A change of state
induced in neurone Z might be expected to affect the surface
condition or membrane at all of the synapses, since the condition
of Z is a factor common to all those membranes. Therefore
a change of state (excitatory or inhibitory) induced in Z by
any of the neurones A, B, C, etc.; playing upon it would enter
as a condition into the nervous transmission at the other synap-
ses from the other collateral neurones. In harmony with this is
the spread of refractory state in the neurones as mentioned above
(p. 122). A change in neurone Z induced by neurone A, playing
upon it, in that case seems to affect its point of nexus with the
other neurones B, C, etc., also. It is conceivable that the phe-
nomena of interference may be based in part at least on such
a condition. The neurone threshold of Z for stimulation through
B will be to some extent a function of events at synapses A Z.

Partial interference. It has to be remembered, however, that
the total final common path, although a functional unity, is often.

IV] THE COMMON PATH 143

especially in compound reflexes, a complex one. It frequently
happens that the set of final paths of one complex reflex is
/ar//|f coextensive with the set of final common paths of another
reflex. With two complex reflexes it can happen that the
reflexes are "allied reflexes" in regard to one part of their
multiple final common path and are antagonistic reflexes in
regard to another part of it We may illustrate this from the
scratch-reflex again. The scratch-reflex was mentioned above
as being unilateral. That is not strictly the case. It is true
that if the right scapular region be stimulated, the right hind
leg scratches ; and if the left scapular region be stimulated the
left hind leg scratches. But if both shoulders be stimulated at
the same time, one or the other leg scratches, but not the two
together. This shows that the scratch-reflex, though at first
sight it appears unilateral, is not strictly so. Suppose the left
shoulder stimulated, the left leg then scratches ; but if the right
leg is examined it is found to present slight steady extension
with some abduction.

This extension of the crossed hind leg which accompanies
the scratching movement of the homonymous hind leg contrib-
utes to support the animal on three legs while it scratches with
the fourth. Suppose stimulation at the left shoulder evoking
the scratching movement of the left leg, and the skin of the
right shoulder then appropriately and strongly stimulated.
This latter stimulus often inhibits the scratching movement in
the opposite leg and starts it in its own.** That is, the stimulus
at the right shoulder not only sets the flexor muscles of the leg
of its own side into scratching action, but it inhibits the flexor
muscles of the opposite leg, because with excitation of the ex-
tensors of the latter leg goes inhibition of their antagonists,
the flexors. The motor neurones of the flexor muscles of the
left leg are part of the final common path not only of the
scratch-reflex of the left shoulder, but also of the scratch-reflex
of the right shoulder ; but in the former case the final common
path is thrown into rhythmic discharging activity, in the latter
case it is steadily inhibited from discharging activity.

Again, the homonymous flexion-reflex of the hind leg (spinal

144 INTERACTION BETWEEN REFLEXES [Lect.

dog) is only the main part of a larger complex reflex which is
bilateral (Fig. 37), and consists of flexion of the same side leg
and extension of the crossed leg (the crossed extension-reflex).
This being so, the mutual relation between the complete scratch-
reflex, e, g. of left foot, and the complete noci-ceptive reflex of
the same foot, is that the homonymous uncrossed parts of each
reflex interfere and are related mutually as antagonistic reflexes ;
but the crossed parts of each reflex coalesce in excitation of
the extensor neurones and inhibition of the flexor neurones of
the right leg, and are related mutually as allied reflexes.

It is the transference of the final common path from the
group of one set of reflexes to another which constitutes the
change which occurs at each step of the orderly sequence of
reaction that we see normally succeed each other in animal
behaviour — leaving aside all question of consciousness in rela-
tion to the sequence. This transference is most obvious when
the sets of reflexes between which the final common path is
exchanged are antagonistic reflexes. Two classes of this kind of
case of specially common occurrence are " alternating reflexes "
and " compensatory reflexes " (Lecture VI.).

Number of common paths. The interaction of reflexes has
been here so far spoken of chiefly in regard to the final
common path, as if the arcs of reflexes met at the final
common path only. But, as stated above, reflex-arcs, espe-
cially the longer ones and those commencing in receptors
far apart, converge and meet to some extent before they
reach their final common path. The receptive neurones, u e.
private paths of the receptors, usually — perhaps always —
reach internuncial paths (J. Hunter, 1770), which in turn
conduct and converge to final paths or to further internuncial
paths. The internuncial paths are thus themselves in various
degrees common to groups of receptive neurones impinging
upon them. They are therefore themselves, to some extent,
common paths?^ There can be little doubt that in the scratch-
reflex the long descending proprio-spinal neurone (Fig. 39 B,
Pa or Py9) is connected not with one but with a whole group of
afferent neurones (private paths) from the scalptor receptors in

rV] WEAK REFLEXES MAY BE NEUTRAL 14S

that part of the skin-field of the scratch-reflex which corresponds
with its own spinal segment Its internuncial path is therefore
common to impubes transmitted to the central organ by many
receptive paths. Again, the structure of the retina (Cajal), olfac-
tory bulb (Cajal)y etc., gives evidence that the conducting fibres
of whole groups of receptors impinge together upon individual
neurones of the next relay. Thalamic neurones form a path
upon which the dorsal-column-fillet and spino-cerebellar-pedun-
cular paths converge. Each internuncial path is therefore
usually, to some extent, a common path,^ just as usually the
receptive neurone, 1. e, private path, itself is common to a small
number of receptors. The ultimate path, therefore, differs from
the intermediate paths only in that it exhibits communism in
the highest degree; it is to distinguish it from internuncial
common paths that it was termed above the final common
path.

Since each instance of convergence of two or more afferent
neurones upon a third, which in regard to them is efferent,
affords, as shown above, an opportunity for coalition or inter-
ference of their actions, each structure at which it occurs is a
mechanism for cthordinaiicn?^

Whatever may be the intimate nature of this mechanism
which gives co-ordination by the formation of a common
path from tributary paths, such common paths exist in ex-
traordinary profusion in the architecture of the gray-centred
nervous system of vertebrates. Two features of that system
indicate this clearly. Enumerations by Donaldson and his co*
workers ^ ^^ show that the afferent fibres (private paths) enter-
ing the human spinal cord three times outnumber the efferent
(final common paths) which leave it Add the cranial nerves
and the so-called optic nerves (in the latter, of course, formation
of common paths having already begun in the retina the afferent
paths are reduced in proportion) and the afferent fibres may be
taken to be five times more numerous than the efferent. The re-
ceptor system bears, therefore, to the efferent paths the relation
of the wide ingress of a funnel to the narrow egress. Further,
each receptor stands in connection not with one efferent only but

146 INTERACTION BETWEEN REFLEXES [Lect.

with many — perhaps widi all, tlKmgh as to some of these only
through synapses of high resistance. The simile to a funnel
wQl therefore be bettered by supposing that within the general
systematic funnel, of which the base is five times wider than the
^^ess, the conducting paths firom each receptor may be repre-
sented as a funnel inverted so that its wider end is more or less
coextensive with the whole plane of emergence of the final
common paths.*** This gives some idea of the enormous forma-
tion of common paths firom tributary paths which must take
place.

Again, there is the accredited fact that under poisoning by
strychnine a muscle can be excited fi-om practicaUy any afferent
nerve in the body ; in other words, that each final common path
is in connection with practicaUy each one of all the receptors
of the body. It is not necessary to accept this literally ; even
if approximately true, it shows the profusion in which common
paths exist

Mntnal indifference between reflexes. In view of such con-
siderations the question arises, Are there in the body no re-
flexes absolutely neutral and indifferent one to another? That
is, in regard to any one reflex using a given common path
cannot another reflex be found which is wholly separate from
it, and neither allied with it nor antagonistic to it? It was
pointed out above that the coalition between scratch-reflexes
gradually decreases as the interval between the receptive points
at the skin surface becomes wider. Whether coalition fades
into mere indifference or passes over into antagonism my own
observations do not answer. But there are reflexes that do
in the spinal dog appear neutral and indifferent to the scratch-
reflex. For instance, a weak reflex of the tail may be ob-
tained without any obvious interference between it and the
scratch-reflex. The stronger two reflexes are, the less do they
remain neutral one to another. Thus, a weak reflex may be
excited from the tail of the spinal dog without interference with
the stepping-reflex of the hind limb ; but a strong reflex (strong
stimulus) in the tail inhibits (Goltz) the stepping-reflex. The
spatial field of response of a reflex increases with its intensity.

rV] THE FINAL COMMON PATH 147

Two reflexes may be neutral to each other when both are weak,
but may interfere when either or both are strong; when weak
they remain " local."

But to show that reflexes may be neutral to each other in
a spinal dog is not evidence that they will be neutral in the
animal with its whole nervous system intact and unmutilated.
It is a cardinal feature of the construction of the higher verte-
brate nervous system that longer indirect reflex-arcs, attached
as extra circuits to the shorter direct ones, all pass through the
brain. With those former intact the number of reflexes neutral
one to another might be fewer. In presence of the arcs of the
gr^dX proficient receptors (Lect. IX) and the brain there can be few
receptive points in the body whose activities are totally indif-
ferent one to another. Correlation of the reflexes from points
widely apart is the crowning contribution of the brain towards
the nervous integration of the individual.

Our conception comes therefore to this. About any final
common path a great number of, or all, the receptive arcs of the
nervous system are arranged and are divisible into sets that do not
act alike upon it It might at first be thought that there would
be simply two such sets, namely, those that excite it and those
that inhibit it. But it must be remembered that we are only
at the beginning of knowledge of differences of time-relations
between different type-reflexes. Thus (Lectures II, III) at the
knee of the spinal dog the time-relations of the extensor-thrust
are vastly different from those of the crossed extension-reflex,
and these again from the extensor tonus that supports the knee-
jerk, and these again from the scratch-reflex, and so on. Of
the reflexes that excite a final common path some evidently
excite it in a manner very different from that in which some
others excite it ; their excitations if concurrent interfere. We
must therefore allow that the sets may be more than two, if the
criterion for distinguishing the sets be interference, /. e, inter-
ruption, displacement, or extinction at the final common path
of one reflex by another.

The final common path is therefore an instrument passive in

148 INTERACnON BETWEEN REFLEXES [Lect.

tbe hands of certain groups of reflex-paAs. I have attempted
to depict this very simply in Fig. 44. There certain type-
reflexes are indicated by lines representing their paths. The
final common path (FC) selected is the motor neurone of the
vasto-<runus of the dog or cat. Reflexes that act as '* allied
reflexes ** on FC are represented as having their terminals joined

PiounB 44. ~ Explanation mainly in text, s stands for scratch-reoeptor, • wad/irt extensor
and flexor muscles of knee respectively.

IV] THE FINAL COMMON PATH 149

K^ether next the final conunon path. Reflexes with excitatory
effect (+ sign) are brought together on the left, those with in-
hibitory (— sign) on the right. Of the reflex pairs formed by
the two reflexes which two symmetrical receptive points, one
right and one left, yield in regard to the flnal common path, one
of the pair only is represented, in order to simplify the diagram.
To have a further indication of the reflexes pla>4ng upon FC,
all that is required is to add to the reflexes indicated in the
diagram for FC, a set of reflexes similar to those given in the dia-
gram for FC', for they must be added if the remaining members
of the right and left reflex pairs from various parts of the body
be taken into account It is noteworthy that in many instances
the end-effect of a spinal reflex initiated from a surface point on
one side is bilateral and takes effect at symmetrical parts, but is
opposite in kind at those two parts, e. g. is inhibition at one
of them, excitation at the other. Hence reflexes initiated from
points corresponding one with the other in the two halves of
the body are commonly antagonistic

150 COMPOUND REFLEXES [Lect.

LECTURE V

COMPOUND REFLEXES: SIMULTANEOUS COMBINATION

Argumetit: Combination of reflexes simultaneously proceeding. Spread
of reflex-response about a focus. Gray matter and lines of reflex
resistance. " Short " reflexes and " long '' reflexes. Rules decipher-
able in the spread of reflex reaction. Pfltiger's '' laws " of spinal
irradiation. The " reflex figure." Variability of reflex result Irra-
diation of a reflex attaches itself to the problem of the simultaneous
combination of reflexes. Co-ordination of reflex result obtains even
when large mixed afferent nerve-trunks are stimulated. The move-
ment excited by stimulation of the motor spinal nerve-root does not
really resemble a movement evoked reflexly or by the will. Extent
of simultaneous combinations of reflexes. Simultaneous stimuli
arrange themselves naturally in constellations in which some com-
ponent is usually of pre-eminent intensity. The resulting compound
reaction has both positive and negative sides.

A LARGE part of co-ordination consists in the orderly com-
bining of reflexes. In studying this co-ordination we have to
deal with and discriminate between simultaneous combinations
and successive combinations of reflexes. We may proceed to
attempt the former problem.

Irradiation. If by appropriate stimulation of the skin of the
foot, say by unipolar faradization of a spot of the plantar skin
of a digit, the ordinary flexion-reflex of the hind limb of the
dog be evoked, the extent of the reflex increases with increase
in the intensity of the stimulus. The reflex-effect spreads over
a larger and larger field, irradiating as it were in various direc-
tions from a focus of reflex-discharge which takes effect on the
limb itself.

The centrifugal discharge elicited by any reflex seems as
regards its spatial distribution to be focussed about a centre
round which its irradiation varies according to circumstance.
In the scratch- reflex the pretibial muscles that dorso-flex the
ankle seem to lie at the focus of the motor discharge. In

V] COMBINATIONS OF REFLEXES 151

the " flexion-reflex," if the reaction evoked is very weak a band
of the deep inner hamstring muscle has often in my experience
seemed the only part of the musculature thrown into action.
On the other hand, when the reflex is evoked with medium
strength it can often be seen that after the reflex (the exciting
stimulation being continued unaltered) has been in progress for
a few seconds, flexion at hip adds itself to the flexion at the
knee (see Fig. 45). And by strong stimulation, strong flexion
at hip occurs together with that at knee and practically from
the very outset. In my experience the condition of "spinal
shock " is very favourable for noting the seat of the focus of
the motor discharge in a reflex, because in that condition it
happens often that the piece of musculature which is at focus
of the discharge is the only one which can be got to give the
reflex-response. It seems possible in this way to determine what
reflex in, for instance, a */ spinal " monkey corresponds with this
or that reflex in a " spinal " dog. In the monkey the severity
and long duration of spinal shock allows merely the focal reply
in the musculature. Thus a feeble tightening of a part of a
hamstring muscle in the " spinal " monkey aflbrds fair evidence,
in response to a stimulus of the foot, that the flexor-reflex — for
the full extent of which one must turn to the " spinal dog " — is
evoked. In man spinal shock seems still more severe and last-
ing than in the monkey. The situation of the weak brief con-
tractions evoked can still reveal which they correspond with
among reflexes better open to study in the lower mammals.

The more intense the spinal reflex — apart from strychnine
and similar convulsant poisoning — the wider, as a general rule,
the extent to which the motor discharge spreads around its focal
area. Thus, as stimulation of the planta causing the flexion-
reflex is increased there is added ^®^» ^^ to the flexion of the
homonymous hind limb extension of the crossed hind limb, then
in the homonymous fore limb extension at elbow and retraction
at shoulder, then at the crossed fore limb flexion at elbow,
extension at wrist, and some protraction at shoulder; also turning
of the head toward the homonymous side, and often opening of
the mouth, also lateral deviation of the tail.

152 COMPOUND REFLEXES [Lect.

According to circumstance, especially according to intensity
of stimulation, the field of end-effect of the flexion-reflex may
vary from a minute field occupying part of a flexor muscle of
the knee to a field including musculature in all four Umbs and
neck and head and tail.

That the reaction should spread in its spatial extent is not
surprising. The afferent neurone on entering the central organ,
the spinal cord, enters a vast network of conduction of paths
interlacing in all directions. A glance at any Wdgert prepara-
tion of the spinal cord shows a tangle of branching nerve-fibres,
the richness and intricacy of which seems practically infinite.
Into this forest the receptive neurone conducts the impulses,
and can itself be traced, breaking up into many divisions that
pass in many directions and to various distances. And this web
of conductive channels into which the centripetal impulses of the
reflex are thus launched is known to be practically a continuum
in the sense that no part of the nervous system is isolated from
the rest " A group of nerve-cells disconnected from the other
nerve-tissues of the body, as the muscles or glands are discon-
nected from each other, would be without physiological sig-
nificance. To understand the physiology of the nervous system
it is important to keep in mind the fact that by histology it is
found to be continuous throughout its entire extent." *• And
there is the generally accredited statement that on exhibition of
strychnine centripetal impulses poured in via any afferent nerve,
excite reflex-discharge over the efferent channels of the whole
nerve-system. This, even if not strictly true, is suflicientiy ap-
proximate to the truth to show the enormous interconnections
between any afferent channel and the congeries of arcs of the

Figure 45. — Maintenance of the scratch-reflex A and the flexion-reflex B respectively under
unipolar faradic stimulation of comparable intensity. The diffuse electrode (anode) was
on the fore limb in each case ; the primary circuit and its rate of interruption was the same,
and the secondary coil of the inductorium remained at the same distance from the pri-
mary. For the scratch-reflex the needle electrode was set in the skin of the loin, for the
flexion-reflex in the plantar skin of the outermost digit. After giving a8 beats the
scratch-reflex died out, having lasted about seven seconds. Further continuation of
the stimulation was cut short as useless after four more seconds. The flexion-reflex, on
the other hand, is in full intensity at the end of the 13th second of continued application
of the stimulus, and its amplitude is perfectly maintained at the end of the 20th, sdthougfa
the reflex is rather tremulant At the 44th second, when it has become more tremulant

154 COMPOUND REFLEXES [Lect.

whole central nervous system. It is therefore not surprising
that the reflex reaction should spread. On the other hand, the
data leave unexplained certain features of the spread. How is
it that the spread, as the reflex is intensified, does not extend
everywhere, as it is said to do in strychnine poisoning? How
is it that in the flexion-reflex, of the cat for instance, the spread
does not extend to the muscles of the pinna of the ear? It is
6asy from certain parts to obtain the brisk reflex retraction of
the pinna. Yet in my experience the stimulation of the foot
that causes the flexion-reflex and all its various irradiations may
be pushed without evoking retraction or other movement of
the pinna. In other words, the irradiations of the reflex occur
along certain lines only and not along others, and the line to
the pinna is of these latter.

Evidently the irradiation from each entrant path tends to run
in certain directions and not in all. This fact is sometimes stated
in the form that gray matter oflers to the entrant path lines of
conduction possessing diflerent degrees of resistance. To say
this merely of course restates the fact in terms suggesting
analogy between nerve-paths and electric circuits. Before the
Golgi and methylene-blue methods had thrown doubt on the
intricate forest of nerve-fibres in the gray matter being a net-
work structurally continuous in all directions, as supposed from
the Gerlach preparations and the universal irradiation under
strychnine, the differences in conductive resistance were attrib-
uted mainly to differences in the length of the network to
be traversed by some reflexes as compared with others. The
longer that path in the gray matter the higher was thought to
be the resistance. Evidence indicating slow travel of impulses
in gray matter was taken as evidence of resistance in gray
matter. In certain reactions the impulses were supposed to
have very long paths of travel in gray matter. Thus, impulses
of pain were supposed to ascend along the spinal gray matter to
the brain. The path of impulses connected with pain does plunge
into the gray matter very soon after entering the spinal cord ;
it then, probably after a short course, emerges into the lateral
white columns, preponderantly of the side crossed from that on

V] NEURONE-THRESHOLD 255

which it entered. This short in-and-out traverse of the spinal
gray matter seems typical of all paths in the gray matter ; they
are probably all quite short** If all synapses lie in the gray
matter, each path where it involves a passage from one link to
another of the neural chain must enter the gray matter to es-
tablish its linkage; it probably soon emerges thence again.

Nenrone-threflhold. But one finds still very generally ex-
pressed the view that the differences of resistance to irradiation
in different directions are referable to different conductive re-
sistance offered by different fibres in the gray matter. The
different resistance seems more probably referable to differences
in the facility of conduction at different synapses. At each
synapse there is a neurone-threshold.^ At each synapse a
small quantity of energy, freed in transmission, acts as a releas-
ing force to a fresh store of energy not along a homogeneous
train of conducting material as in a nerve-fibre pure and simple,
but across a barrier which whether lower or higher is always to
some extent a barrier. There is abundant evidence that different
synapses differ from one another. That neurones should differ
in the threshold value of the stimulus necessary to excite them
seems only natural. The arguments adduced by Goldscheider
point in this same direction. Many of the phenomena consid-
ered in the first three lectures are easiest explicable by such
differences. The distinctions between different synapses in re-
gard to ease of alteration by strychnine and by tetanus toxin
emphasize this probability further. On this view the fact that
irradiation of a reflex reaction spreads along certain conductive
arcs more readily than along others, can be schematically figured
as in the diagram (Fig. 46). A receptive neurone A enters the
cord and forms synaptic connections with three neurones, the
neurone-threshold at the synapse with one of the neurones is
higher than that at the synapses with the others. The threshold
heights (resistances) are represented by whole numbers, two and
one respectively. Each of the int];aspinal neurones in its turn
forms two synaptic connections with two neurones, and in these
cases also the thresholds at the synapses are of different heights,
numerically, two and one respectively. On the view that the

IAH£ McCJCAL LfBRAW Of
STATiFOaO UHWi«SKt

PM.0 100, ai»«»*^

156

COMPOUND REFLEXES

[Lect.

FiGURB 46. ~ Explanatioa in text.

action of one neurone upon the next is that of a releasing force
liberating a potential system across a barrier whose resistance
we do not exactly know, it is impossible to predict how the re-
sistance will sum along the whole conductive chain. It is clear
that although the total resistance of the reflex-arc A B may be
numerically represented by i, the resistance along A D need
not, on the numerical values assigned to the synapses in the
diagram (Fig. 46), sum to the value 4. Yet it is also clear that
the threshold for any whole arc cannot be lower than the
highest individual threshold in it. Further, the individual
thresholds will tend to sum, for an excitation of neurone A
just sufficient to excite neurone a^ is hardly likely to excite
a sufficiently to overcome the threshold of synapse a D. Thus,
with even small grades of difference of threshold at different
synapses, large diff'erences in the conductive facility of different
reflex-arcs can be established.

Similarly, an additive influence of the threshold will make a
reflex-chain consisting of several neurones offer caeteris paribus
higher resistance than a chain of fewer neurones. The diagram
is therefore in accord with the rule that the reflex-chains which
conduct to parts segmentally distant require generally intenser
stimulation to excite them than do merely local arcs.

V] IRRADIATION 157

Short and long reflexes. For many purposes of description
it is convenient to divide reflexes into " short " and " long." **

The cord may, in its relation to the receptive surface and
skeletal musculature, be considered divisible into right and left
lateral halves, each subdivisible into regions of neck (cervical,
including pinna), fore limb (brachial), trunk (thoracic), hind limb
(crural), and tail (caudal). A reflex action in which the stimu-
lus applied to a receptive area in one of the above regions
evokes a reaction in the musculature of another of the regions
is conveniently called a long spinal reflex. A reflex reaction in
which the muscular reply occurs in the same region as the
application of the stimulus is conveniently called a short spinal
reflex. Short spinal reflexes are, as a rule, more easily and
regularly elicitable than are long spinal reflexes. It might
further be convenient to allocate hard-and-fast boundaries to
these regions, but such limits would of necessity be artificial
and arbitrary. The scope of the delimitation is indicated and
its purpose better served by comparison with one retina, say of
the bird, the one lateral half of the skin corresponding with one
retina ; one optic nerve would correspond with the lateral half of
the spinal cord and bulb. Between these comparable surfaces a
difference exists, in that the receptive field of the skin, unlike
the retinal, has instead of one (or two, cf. Kalischer) '^^ focal
region of concentrated responsiveness, several such foci, e. g.
the relatively highly responsive skin at the apex of each limb.
As the retina has muscles at call, so also the skin. The close-
ness of nexus between a retinal point and the visual muscula-
ture is graduate in degree, e. g, most close for the muscles of its
own bulbus, next for those of the contralateral, then for the
neck muscles, etc. Similarly there are degrees of nexal closeness
between a point of skin and the related musculature : its connec-
tion is most close with muscles of its own limb, next with those
of another limb or other region. The main interest of direc-
tion of nervous irradiation per se — apart fi-om light it may
incidently cast upon the integrative work of the nervous system
— lies in its elucidation of the machinery for working sentient
surfaces. That the skin is a region which morphologically

iS8 ^ COMPOUND REFLEXES [Lect.

considered is composed of a segmental series seems to have
been allowed greater weight in the estimation of its receptive
functions than is fully justified, at least in the higher vertebrata.
That its segmental innervation demonstrably limits existing
reflex spinal functions in the mammal has not been shown.^*

Roles observed in the spread of impulses in spinal reflexes.
Regarding short spinal reflexes, and the directions taken by the
examples of intraspinal irradition which they furnish, it is possi-
ble to make certain general statements.^

I. Broadly speaking, the degree of reflex spinal intimacy
between afferent and efferent spinal roots varies directly as their
segmental proximity. Thus excitation of the central side of a
severed thoracic root, e. g. seventh, evokes with especial ease
contraction of muscles or parts of muscles innervated by the
corresponding motor roots, and next easily muscles innervated
by the next adjacent motor roots. The spread of short spinal
reflexes in many instances seems to be rather easier tailward
than headward. This may be related with the oblique correla-
tion that so largely holds between the distribution of the aflerent
root in the skin and the distribution of the efferent root in the
underlying muscles.

II. Taken generally, for each afferent root there exists in
immediate proximity to its own place of entrance in the cord
(e. g, in its own segment^ a reflex motor path of as low a thresh-
old and of as high potency as any open to it anywhere.
Further, in response to excitation even approximately mini-
mal in intensity a single afferent root, or a single fllament of
a single root, evokes a spinal discharge of centrifugal impulses
through more than one efferent root, /. e. the discharge is
plurisegmental. And this holds especially in the limb regions.
In the limb region the nerye root is therefore a morphological
aggregate of nerve-fibres, rather than a functionally determined
assortment of impulse-paths. The view that the efferent spinal
root is a functional assemblage of nerve-fibres is certainly erro-
neous. The formation of functional collections of nerve paths
(peripheral nerve-trunks) out of morphological collections (nerve
roots) seems to be the meaning of the limb-plexuses.

V] INHIBITION REVEALS IRRADIATION 159

III. Motor mechanisms for the skeletal musculature lying
in the same region of the cord, and in the selfsame spinal seg-
ment, exhibit markedly unequal accessibilty to the local afferetit
channels as judged by pressor effects. For example, if pressor
effects only, and the primary phase only, of the reflex movement
be considered, the flexors of the homonymous knee and the
extensors of the contralateral are in many animals much more
accessible than the extensors of the homonymous and the
flexors of the contralateral. Inasmuch as at many joints the
flexors and extensors are both innervated by motor-fibres con-
tained in one and the same efferent root, it follows that the
reflex movement obtained by excitation of an afferent root in
many cases is quite dissimilar from the movement obtained by
excitation of the corresponding efferent root, in spite of the rule
of segmental proximity.

It is necessary to insert the qualification '* pressor " before
" effects " (" reciprocal innervation "). It is only in regard to
pressor effect that the above statement holds for such contrasted
neurones as those of " extensors " and " flexors." I have stated
the rule in this way because more in conformity with the oft-
quoted rule of spinal reflexes coming to us from Ludwig's labor-
atory, which insisted on the rarity or impossibility of obtaining
hind-limb extension as a primary homonymous spinal reflex.
But how easy and direct is really the reflex nexus between the
receptive surface of the limb and its extensor muscles, e. g. at
knee, is shown by nothing better than by giving a small dose of
strychnine. That alkaloid has, as has been mentioned, the
property of converting spinal reflex inhibition into excitation.
The same stimulus which normally reflexly excites the knee-
flexors to contraction is seen after the strychnine to excite the
knee-extensors to contraction. The reflex inhibition of the ex-
tensors which was previously the reflex-effect is more difficult
to observe, but by turning it into excitation the facility of the
reflex nexus with the extensors is found to be as close as with
the flexors. Therefore, in the rule before us, if inhibition and
excitation are both — as they should be — counted as evidence
of the reflex nexus, then the reflex nexus with the homony-

i6o COMPOUND REFLEXES [Lect.

mous knee-extensors and with the crossed knee-flexors, is as
close as with the homonymous knee-flexors and the crossed
knee-extensors.

In the question, therefore, that was put above, How is it that
the spread of a reflex reaction, when the reflex is intensified,
does not extend to all parts, as it is said to do in strychnine
poisoning? there are two different things involved. It does not
spread to some parts because, as argued above, an additive
synaptic resistance intervenes across the potentially conductive
path. An instance of such a path was given. But the absence
of apparent irradiation to certain others is for a different reason.
Keeping to the flexion-reflex as elicited by unipolar faradization
of the plantar skin of a digit as illustration, the instance of the
knee-extensor may be taken. However intensely the stimula-
tion may be pushed, although the reflex reaction is thereby
more and more intensified, contraction of the knee-extensor does
not result, — but for a wholly different reason than that suggested
for the absence of spread of the reflex from the leg to the pinna.
The muscles of the pinna, in my experience, do not at all easily
become involved in the reaction; but the extensor muscle of
the knee is really involved in the reaction from the beginning,
only it is involved in a way that escapes observation unless
special means be taken to reveal it. The reflex-effect upon
it takes the form of an inhibition of the efferent path just
central to its motor neurone, an inhibitory block, which in pro-
portion as the intensity of the exciting stimulus of the reflex
is increased simply becomes itself the more intense. There is
no evidence that this can be broken down and converted into
excitation by merely increasing the intensity of the stimulus
that is evoking it. On the other hand, as shown above, strych-
nine and tetanus toxin convert it into excitation, and that is
one reason why strychnine seems to increase the spread of re-
flexes so greatly; but in this case the increase of spread which
that drug appears to cause is really merely apparent. The reflex-
effect was there already, but had another form of expression.

IV. The groups of motor nerve-cells contemporaneously dis-
charged by spinal reflex action innervate synergic and not antergic

V] IRRADIATION IN LONG REFLEXES i6i

muscles. This is the reverse of the view that since Winslow
and Duchenne^ has been common doctrine concerning mus-
cular co-ordination. It controverts an argument adduced for the
view that the limb movement evoked by excitation of an efferent
root represents a highly co-ordinate functional synergism.*®* ^
The spinal reflex in its intraspinal irradiation develops a com-
bined movement and synthesizes a muscular harmony.

V. It follows almost as a corollary from this, and from the
rule of spatial proximity (p. 158), that the spinal reflex movement
elicitable in and from any one spinal region will exhibit much
uniformity despite considerable variety of the locus of incidence
of the exciting stimulus. Approximately the same movement^
e. g. in the hind-limb flexion of the three great joints, will result,
whatever piece of the limb surface be irritated. The locus of
incidence of the Stimulus will only influence the character of the
general movement executed by the limb musculature, in so far
that the flexion will tend to predominantly occur at that joint
the flexor muscles of which are innervated by motor cells seg-
mentally near to the entrance of the afferent fibres from the
particular piece of skin the seat of application of the stimulus.
Another way of expressing this rule is to say that the receptive
field of a " type-reflex " is usually of plurisegmental cutaneous
extent.

Part of the question of spatial distribution of the motor dis-
charge of a spinal reflex has long been studied, and a funda-
mental contribution to knowledge of it was made by Pfliiger.*'
His inductions were based chiefly upon observations on the frog
and on the records of clinical cases of spinal lesion in man.
They were drawn up in the form of four " laws."

It was regarding the course of irradiation in long spinal
reflexes, namely those spinal reflexes that initiated from one of
the above mentioned spinal regions spread over into others that
Pfliiger,^ in 1853, formulated his four " laws." These " Laws"
have for many years been widely accepted.^® They are stated
as follows : —

I. The law of homonymous conduction for unilateral re-
flexes. If a stimulus applied to a sensory nerve provokes mus-

i62 COMPOUND REFLEXES [Lect.

cular movements solely on one side of the body, that move-
ment occurs under all circumstances and without exception
on the same side of the body as the seat of application of the
stimulus.

If, as is clear from the context in the original paper, by
movement on the same side is meant contraction of muscles on
the same side, this statement does not in reality very completely
express the facts.^ It is in part an outcome of the rule of
spatial proximity, but certain cases which conform to the latter
yet offer striking exception to the former ; for instance, when the
skin of the tail is stimulated on one side the organ is very fre-
quently moved towards the opposite, and this in a great number
of classes, from fish to mammal inclusive.

2. The law of bilateral symmetry of the reflex action.
When the change produced in the central organ by excitation
of a sensory nerve has already evoked unilateral reflex, it, if it
spreads farther, excites in the contralateral half of the cord
only those motor mechanisms which are symmetrical with those
already excited in the homonymous half of the cord. This
statement, although true of a number of instances, fails to con-
form with fact in many, even perhaps the majority.

The important cross-reflex from the hind limb of the bird
and mammal does not conform to it; so similarly with the fore
limb. The asymmetry of the crossed reflexes of the limbs is
important because probably connected with the fundamental
co-ordination of muscles for progression. Again, the wag-
reflex of the tail, and a reflex I have called the " torticollis
reflex"^®* (cervical region), afford important exceptions to
the ** law." And many other exceptions can be found. In the
spinal rabbit, on the other hand, and less often in the dog, the
crossed reflex from one hind limb to the other is sometimes not
an asymmetrical movement, but a symmetrical one : this seems
to stand in obvious relation to the hopping mode of progression
of the animal.

3. The law of unequal intensity of bilateral reflexes. When
the excitation of a sensory nerve elicits reflex action involving
both halves of the body, and the action is unequal on the two

V] THE REFLEX FIGURE 163

sides, the side of stronger contractions is always .that homony-
mous with the seat of application of the stimulus.

This statement is in conformity with a number of instances.
The following are examples. When bilateral retraction of the
abdomen is excited from the skin of the chest, the contralateral
retraction is much the less marked : in the bilateral protraction
of the " whiskers " (cat, rabbit, dog) on the excitation of the
skin of the face, the crossed movement is the less ample. An
interesting illustration,^** because it involves inhibitory as
well as pressor influence, can be demonstrated in the spinal cat
or dog thus: — The animal resting comfortably on its back, if
one hind paw be pressed that leg will be flexed at hip, knee,
and ankle, in accordance with the rules laid down on p. 158, and
if the stimulus be strong, or the reflex excitability good, the
fellow hind limb will be extended. If instead of one hind paw
both hind paws be pressed, both hind limbs are simultane-
ously flexed, and there is no trace of extension (Fig. 66, Lect.
VI. p. 225). The homonymous reflex is prepotent, therefore,
and inhibits the crossed reflex.

But there are also a number of exceptions to this " law,"
among others, the abduction of the tail from the side stimulated
already referred to.

4. The fourth of Pfliiger's classical " laws " of spinal reflex
action states that with associated spinal reflex centres the irradi-
ation spreads more easily in the direction toward than in the
direction away from the head.

My own experience in the mammal is far from completely
accordant with this statement: in, I think, the majority of in-
stances, irradiation has spread more easily down than up the
cord.^» **»>" It is easy to obtain reflex movements of the limbs
and tail by excitation of the skin of the pinna, whereas the
reverse is rare. To elicit by excitation of the hind limb a move-
ment of the fore limb, is more diflicult than by excitation of the
fore limb to elicit movement of the hind limb. To elicit move-
ment of the tail by excitation of the fore limb is easier than to
move the fore limb by excitation of the tail. The irradiation
has in my experience been easier across the cord from hind

i64

[Lect.

FiGURS 47. — a. Position of animal after tnmsection at
h. Position under decerebrate rigidity.
c. Change of attitude from b evoked by

scrifimrnu.
of left pn

limb to hind limb than from hind limb to fore limb ; but it is often
easier down the cord from fore limb to hind limb than across
from fore limb to fore limb. In such reflexes also as the " shake"
reflex (a reflex in which the trunk is shaken, as when a dog comes
out of water), which implicate the trunk more than the limbs, the
radiation is away from the head, for it is well obtained as a rump
reflex when the skin of the shoulder is the part rubbed. In the
" scratch-reflex," too, the skin stimulus is applied far headward
of the region of the muscular contraction evoked.

These so-called " laws " of reflex irradiation were so generally
accepted as to obtain a doctrinal eminence which they hardly
merit. It seems here less profitable to attempt adapting them
to better fit the observed facts than to briefly describe the
salient features of the long spinal reflexes as exhibited in an
ordinary experiment on the spinal mammal.

The reflex figure. When the animal is supported freely
from above, with its spine horizontal and the limbs pendant,
a point that early strikes the observer is that there are ten
areas whence bulbo-spinal reflexes employing skeletal muscula-
ture can be provoked with pre-eminent facility. These areas

THE REFLEX FIGURE
a b c

165

Figure 4S. — a. Position nnder decerebrate rigidity.

^. Change of attitude from a evoked by stimulation of left fore foot.
c. Change of attitude from a evoked by stimulation of left hind foot.

are the soles, the palms, the pinnae, the mouth, the snout, and
the tail and cloacal region. It is significant that nine of these
areas are those which possess the greatest range of motility if
the axis of the animal be considered fixed. Stimulation at any
one of these areas causes a particular attitude — a refiex figure
— to be struck. From the pinna is excited movement of each
limb, the neck, the tail, and the trunk (Fig. 47). The irradia-
tion from this reflexigenous area usually presents the following
order: (i) Neck and homonymous fore limb, (2) homonymous
hind limb, (3) tail and trunk on both sides, (4) contralateral
hind limb, (5) contralateral fore limb. From the fore foot (Fig.
48) can be excited besides movements in the fore limb itself,
movements in the other limbs and tail. The facility of radia-
tion is usually in the following descending series : ( i ) homony-
mous hind limb and the tail, (2) crossed hind limb, (3) crossed
fore limb. The relative facility of spread of the reaction to the
crossed fore limb seems subject to much variation. In the frog
the path between the two fore limbs is, especially in the breed-
ing season, very open and facile. In the cat and monkey it
seems to be much more open in the bulbo-spinal than in the
spinal animal. From the hind foot (Fig. 48) the frequency

i66 COMPOUND REFLEXES [Lect.

and ease of irradiation into other spinal regions usually appears to
exist in the foUowing order: (i) extension of crossed hind limb
and taiU (2) extension of homonymous fore limb, (3) flexion of
crossed fore limb. The £su:ility of " spread " from one lateral
half of the cord to the other is very disdmilar at different
levels of the cord. It is particularly easy in certain parts of
the tail region. Motor mechanisms which are yoked together
are for the most part, as with the flexion-extension mechanisms
of the hip and knee, of an as)anmetrical kind. In the hind-limb
region the crossed irradiation is also fairly free, and largely
connects asymmetrical muscle-groups; but one of the most
facile and persistent of all bilateral reflexes resulting from uni-
lateral stimulation is the adduction of both thighs, a bilaterally
symmetrical movement.

In the trunk the spread across the median plane is most free
for skin reflexes excited from near the midline ; this is seen in
the venter of the frog ; the yoking is of bilaterally S3mEmietrical
muscles. To excite movement of one fore limb from the other
is less easy than to excite one hind limb from the other, at least
in many animals. In the neck region irradiation across the
median sagittal plane is fairly easy, and the yoking connects in
large part asymmetrical muscles.

On the whole, the " long " spinal reflexes are more variable
and less validly predictable than the short They vary in a
series of experiments, not only as to order of relative facility of
direction of irradiation, but as to the sense of the movement
elicited at the joint, whatever it may be, to which irradiation
extends. Not unfrequently a region to which the reflex usu-
ally irradiates is altogether omitted, and omitted consistently
throughout the whole of a lengthy experiment, although
the spinal region in question has, so far as known, suffered
no damage, nor indeed been directly implicated in any of the
procedure. Thus, excitation of the skin of the neck or pinna
will sometimes spread back along the cord and produce move-
ment in the tail, or in the hind limbs, and in doing so pass by
the fore limbs without evoking a twitch in either of them. The
motor mechanisms of the fore limb thus skipped over may show

V] THE REFLEX FIGURE 167

no sign when examined by the local reflexes of being less amen-
able than usual.

The inconstancy of the irradiation as to the kind of movement
produced, e, g, whether it flex or extend a limb, is different in
different reflexes. The irradiations from the " drawing-up reflex "
of the hind foot (i. e, the flexion-reflex) have great constancy;
the irradiation is shown in Fig. 48 ; to the figure the turning of
the head to the homonymous side may be added ; the irradia-
tion from the fore-foot reflex is less regular ; sometimes flexion
at crossed knee, sometimes extension. The variation is from
experiment to experiment, not during a single experiment

There is some evidence that the influence exerted on a com-
mon path by one and the same afferent arc may not always be
of the same kind. It is true that the regularity with which the
same end-result appears and reappears in observations dealing
with certain reflexes is very great, and inclines the observer to
regard the reaction of the reflex-arc as perfectly constant But
that is not equally clear of all the reflexes. Instances of incon-
stancy seem to occur in some reflex-arcs, and these suggest the
possibility that in some cases one and the same afferent arc may
exert on a final common path even reverse effects at different
times ; in other words, under different conditions. The effect of
stimuli to the pinna of the '' bulbo-spinal " cat seems sometimes
to be flexion of the hind limb, sometimes extension of that
limb. Stimulation of the afferent nerve of a part of the vasto-
crureus muscle is often inhibition of the rest of that muscle,
but sometimes not In my experience these results, though
variable from experiment to experiment, do not vary during
the same experiment Again, the afferent nerve, stimulation
of which excites reflex rise of arterial pressure under curare,
is known to yield reflex fall of pressure under chloraL It must
be admitted that here other explanations are indeed possi-
ble, besides the supposition that the kind of influence excited
by the afferent arc on the efferent path has changed

Irradiation of a reflex attaches itself to the problem of the
simultaneous combination of reflexes. It does so because it
affords clear evidence that by irradiation a reflex assumes use

i68 COMPOUND REFLEXES [Lect.

of a number of final common paths which do not in the first
instance belong to it, but belong in the first instance rather
to reflexes arising in their own immediate segmental locality.
From them a " reflex figure " is formed. Thus, by irradi-
ation, the flexion-reflex of the right planta causes reflex-
discharge down the motor nerve of the cubital extensor (part
of triceps) of the homonymous fore limb. But this reflex
motor discharge to the cubital extensor is more easily excited
by stimulation of the left fore-paw. Again the flexion-reflex
of the right planta, if strong, will irradiate as motor discharge
into the flexors of the left elbow, but the reflex motor dis-
charge into the flexors of the left elbow is much more easily
obtained by stimulation of the left fore paw ; so that the irra-
diation welds into a single combined reflex effects belonging
primarily, as it were, to several reflexes. But the reflexes
whose effects are thus combined are always reflexes of what
was termed above *' allied " relation. Thus, if a stimulus ex-
citing the flexion-reflex from the right planta be just subliminal
for evoking the irradiation to the homonymous cubital extensor
and a stimulus be applied to the left fore paw of an intensity
by itself just subliminal for provoking crossed elbow-extension,
the two stimuli applied simultaneously mutually facilitate and
the reflex of the right fore limb results.

Moreover, it seems to me significant that the irradiation
extends rdLth^r per saltum than gradatim. As the flexion-reflex
is continued, flexion at hip (Fig. 45, p. 153) can be seen to add
itself almost suddenly to flexion already in progress at knee.

Romanes "^ writes of irradiation in Medusa as follows : " It
is not difficult to obtain a series of lithocysts connected in
such a manner that the resistance offered to the passage of the
waves by a certain width of the junction-tissue is such as just to
allow the residuum of the contraction wave which emanates from
one lithocyst to reach the adjacent lithocyst, thus causing it
to originate another wave, which in turn is just able to pass to
the next lithocyst in the series, and so on, each lithocyst acting
in turn like a reinforcing battery to the passage of the contrac-
tion wave. Now this, I think, sufficiently explains the mech-

V] SIMULTANEOUS COMBINATION 169

anisoi of ganglionic action in those cases where one or more
lithocysts are prepotent over the others; that is to say, the
prepotent lithocyst first originates a contraction wave which is
then successively reinforced by all the other lithocysts during
its passage round the swimming-bell." If we read for "pre-
potent lithocyst" the "exciting external stimulus" of the arc
primarily stimulated and for the other lithocysts the other arcs
to which the excitement of the one primarily stimulated extends,
it seems to me we have in the above description of Aurelta
aurita a description that applies well to the process of reflex
irradiation in the central nervous organ of vertebrates.

It may be objected that in the case of Medusa the wave of
contraction is reinforced by, on reaching the lithocyst, initiating
through that a new reflex which reinforces the one already in
progress; whereas in the spread of the flexion-reflex to the
reflex-arcs of the fore limb, the reaction does not initiate in
these latter anything that can be called a new reflex because the
reaction in them is not excited through their local receptors,
the normal point of departure for their reflexes. That is a dif-
ference certainly, and a significant one. But it does not vitiate
the analogy from the point of view under consideration now.
In Medusa the irradiation of the reflex is in its propagation
reinforced at the certain points mentioned by the reaction in
its spread exciting a new neurone, attached to its path, across a
definite threshold resistance. In Medusa the threshold lies in
Romanes' view at the receptor organ. In the irradiation of
the flexion-reflex the reaction also breaks at certain points into
new arcs across a threshold resistance, and once over the thresh-
old, propagates itself along those arcs, as evidenced by the
movements produced. It is in accordance with that mode of
propagation that the irradiation of the reflex appears to occur
per saltum rather than gradatim (Fig. 45). Here again it is
noteworthy that the places of reinforcement in the spread
of the reaction which are peripheral in Medusa are central in
the vertebrate; in other words, just as refractory state, inhibi-
tion, interference, etc., which are peripheral in Medusa are
central in the vertebrate, so with this latter instance of "rein-

I70 COMPOUND REFLEXES [Lect.

forcement." The reason which seemed obvious before, applies
in the present instance also, and is the same as that which ex-
plains the centrality of the central nervous system itself (Lect. IX).
It is not only when the spot stimulated is a receptor that the
reflex shows itself co-ordinate. The stimulation of the central
end of any even large afferent nerve-trunk, or even the central
end of a whole spinal afferent root, evokes reflexly a movement
that is co-ordinate. This result is familiar and commonplace,
but it is also remarkable.

For stimulation of the central ends of that vast medley of
afferents, from different sources and of various species, collected
together in one afferent spinal root {e.g. eighth cervical) to evoke
no inharmonious confusion of various reflexes, such as the com-
ponent fibres in it must, taken individually, represent, but one
allied group of harmonious reflexes, is a result that, though
regularly obtainable, is surely not what the observer might have
expected would occur. Stimulation of the central end of the
tibial nerve behind the ankle causes flexion at knee, hip, and
ankle, in which the normal inhibitions of extensor muscles ac-
company the contractions of the flexor muscles. Yet in that
nerve are included the afferent fibres from the planta which
evolve the powerful extension-reflex, the extensor-thrust How
is it that this reflex does not appear conjoined with the flexion-
reflex produced by the other afferent fibres in the posterior root
or tibial nerve? The principle of interference of antagonistic
reflexes central to the mouth of the common path on which both
embouch precludes such confusion of reflexes. Such simulta-
neous combination of flexion and extension reflexes would be
inco-ordination. The formation of a common path from tribu-
tary paths is a mechanism ensuring co-ordination against that.
The stimulation of the nerve containing admixed receptive paths
of different and antagonistic reflexes excites reflexly through
the central organ an effect in the skeletal musculature which is
co-ordinate and synergic. It is as though a solvent were simul-
taneously supersaturated with two crystalloids, and as though
when a pair of these crystals were simultaneously dropped into
the solution the crystallization out took place of one salt or of

V] REFLEX FIGURE INVOLVES INHIBITION 171

the other, but not of both together. What happens resembles
what occurs when there is presented to the eye one of the plane
figures suggesting visual perspective, but equivocally in either
of the two ways ; the whole of the beaker or of the flight of
steps appears set in one way or in the other, never partly in
one way, partly in the other. And this is a really germane
analogy.

The mode of preclusion of the antagonistic reflexes seems
so closely akin to the process which occurs when one reflex in
its supervention on another dispossesses an antagonistic reflex
from the common path that its discussion may be deferred until
treating of the co-ordination of successive reflexes. But it is
obvious that in the irradiation of a reflex so as to produce a
combined movement of remote parts we have really a syn-
thesis of simultaneous reflexes. The parts of the reflex finding
simultaneous expression in the efferent paths of other reflexes
are combined by a process which tends to exclude the antago-
nistic reflex for each component part. It is obvious that while
" allied " reflexes can be compounded together both in simulta-
neous and successive combination, antagonistic reflexes can be
combined only in successive combination.

The collection of fibres in a motor spinal root does not repre-
sent a "reflex figore." The supporters of the view that the
motor-fibres gathered together in a motor spinal root form a
collection assorted so as to represent the fibres that normally
are excited together in willed and other actions adduce the fact
that antagonistic muscles are together thrpwn into contraction
on exciting this or that spinal motor nerve root supplying a limb
— e. g.^ the arm. This fact, as I pointed out some years ^ ago,
is in reality one of the clearest evidences that their view is
erroneous, because most commonly in normal movements the
antagonistic muscles, far from being thrown into contraction
together, are reciprocally innervated, one antagonist being made
to contract and the other to relax. Further, far from a normal
action ever throwing into activity all the motor-fibres of a
single motor root, still less using that one root thus solely
without other roots, in reality the evidence is that in all normal

172 COMPOUND REFLEXES [Lect,

actions, reflex or voluntary, especially in the limb regions, the
centrifugal discharge to the muscles takes place through scattered
motor-fibres contained in several roots, even when the action pro-
voked and the movement effected are weak. Outside the limb
region, those who argue that the aggregation of motor-fibres in
each efferent spinal root represents some definite synergic con-
traction of muscles for a co-ordinate movement must disregard
the observation of Newell Martin, and Hartwell^ that in the
normal breathing of the dog the action that goes forward in the
internal and external intercostal muscles is alternating in them.
One relaxes as the other contracts ; yet both the external and
the internal intercostal muscle of the space receive their motor
supply from one and the same motor spinal root

Those who hold the view that the assortment of the fibres
of the motor root is functional, state that the movements which
result from stimulation of these individual roots in the brachial
region are not mere contractions more or less strong of various
muscles, but are a highly co-ordinated functional synergy in
each case. To this one may reply that the mere superficial
resemblance of the position assumed by the limb, to one
of the manifold positions assumed by it in the normal activity,
is a slender analogy. The hind limbs of a frog, when it tries
to climb the side of the bell-jar which confines it, assume an
attitude of extreme extension, in outward semblance like that
of a strychnine cramp, or that due to excitation of the eighth
root ; but is it permissible from that resemblance to argue that
excitation of the eighth root produces a co-ordinate move-
ment of the limb? The same analogy would argue that the
strychnic cramp is also a co-ordinate movement of the limb,
whereas it is definitely known to be inco-ordinate.

Myself I have not been struck by resemblance between the
movement produced by excitation of the motor spinal root of a
limb-plexus and the co-ordinate normal movements of the limb.
It may be urged that in order to obtain the resemblance the
excitation employed must be strong, so as to bring into full
action every component of the complex entity of the root.
When I have done this the resulting movement has seemed to

V] FIGURE NOT GIVEN BY MOTOR ROOT 173

me, e.g. in the case of the sixth subthoracic root of the monkey,
like a strychnine cramp rather than a movement of co-ordinate
adjustment. If, on the other hand, it be urged that minimal
excitation must be used, I have not been able to obtain in that
way any more obvious relation to a co-ordinate movement. For
instance, in the lumbo-spinal region of the monkey, excitation
when just effective induces through the ninth subthoracic motor
root abduction of the tail and flexion of the toes without any
movement elsewhere. Similar excitation of the eighth motor
root induces reflexion of minimus and hallux without the inter-
vening digits, not infrequently accompanied by pursing of the
anus. Such combinations strike the observer as bizarre and give
little suggestion of the bringing into play of a highly co-ordinate
functional synergy.

On the view that the compound muscular contraction
obtained by excitation of one whole motor root is highly co-
ordinated and due to a group of contractions combined in
accordance with some plan for a functional result, it might be
expected that the severance of one such motor root in a limb
region would result in loss of some particular co-ordinate move-
ment, and that the disappearance of that movement might be
fairly clearly detectible. I was unable to detect such a result
and saw no evidence in support of its existence. The severance
of a single motor root seemed to produce not the complete loss
of any one particular movement, but a weakened condition of
many movements. Even when two of the motor spinal roots
were cut the effect on the movements of the limb was rather
weakness of movement than inco-ordination of movement. When
the number of consecutive nerves cut was more than two there
appeared limitation of the range of movement by loss in some
particular direction. I inferred from my experiments that the
mechanism for specific movement of each part of a limb {e, g.
a digit) is so placed in the cord that the efferent fibres debouch-
ing from it into the motor roots pass via many root filaments
and via at least two, usually more, spinal roots. Thus there is
not in any one motor root filament, nor even in any one motor
root, a perfect representation of any one movement, but only an

174 COMPOUND REFLEXES [Lect.

imperfect representation of several adjacent local movements,
though not for each equally imperfect

Against the view that the aggregation of efferent fibres in a
motor spinal root represents a functionally co-operative collection
is the fact that between them there may intervene a high resist-
ance ; that is to say, afferent impulses that easily throw some
of them into action have great difficulty in throwing others of
them into action. Thus, the ninth subthoracic motor root of the
monkey sometimes contains efferent fibres to the urinary bladder
and to the muscles of the leg. It is easy to obtain a reflex on
the latter through the afferent root of the ninth, but relatively
difficult to obtain a reflex upon the bladder. The synergetic
view of the character of the collection of fibres in the spinal
motor root presupposes or infers that mere spatial juxtaposi-
tion possesses curiously high value for spinal co-ordination ; but
are the separate spinal and bulbar elements of the respiratory
centre less perfectly associated in function because in the neural
axis they are placed apart? Likeness of quality rather than
proximity in space insures the harmony of their reactions. I
conclude, therefore, that the collection of fibres in a spinal
motor root is not a functional collection in the sense that it is
representative of any co-ordination.^

The receptive field of a reflex does not oonform with the fleld
of distribution of an afferent spinal root. Similarly with the
afferent root. The distribution in the skin of any afferent root
does not correspond with the receptive field of any cutaneous
reflex. The skin-field of the scratch-reflex is made up of parts
of the skin-fields of many adjacent spinal roots (compare Fig.
49 with Fig. 39, p. 121). The skin-field of the flexion-reflex
of the hind limb similarly; that of the fore limb similarly.
Even the relatively limited receptive field ^^ of the extensor-
thrust is a patch into which parts of the fields of at least two
spinal roots enter. Nor do the limits of the receptive fields of
cutaneous reflexes respect the limits of spinal root skin-fields.
Again, the afferent nerve of the extensor cruris muscle evokes
the same reflex as does that of the flexor muscle, yet the two
belong to wholly different spinal nerves.

V] REINFORCEMENT 175

Reiiiforoement« The overflow of reflex action into channels
belonging primarily to other reflex-arcs than that under stimula-
tion leads to the production by the single stimulus of a wide»
compound reflex which is tantamount in effect to a simultaneous
combination of several allied reflexes.

When in the spinal animal the one fore foot is stimulated,
flexion of the hind leg of the crossed side is often obtained.
Stimulation of that hind foot itself also causes a like reflex of
that limb. When these two are concurrentiy stimulated, the
flexion movement is obtained more easily than from either
singly. These widely separate reflex-arcs therefore reinforce
one another in their action on the final common paths they
possess in common. Similarly with certain reflex-arcs arising
from the sldn of the pinna of the crossed ear. In them excita-
tion reinforces that of the just mentioned arcs from the fore foot
and opposite hind foot.

This reinforcement is significant of the solidarity of the whole
spinal mechanism ; but significant of more extensive solidarity
still are results observed by Exner.^*»^ A sound conveyed
to the ear of a chloralized rabbit, he found, increased the am-
plitude of a reflex movement of the foot, induced by the stimulus
applied to the foot a moment later. Sternberg has studied
similar summation of reflexes.^^ The same significance proba-
bly attaches to the influence of various precurrent stimuli on
the knee-jerk in man, studied by Jendrassik,^^ Mitchell and
Lewis,^ Lombard,^ and by Bowditch and Warren.^ In these
cases of course cerebral as well as subcerebral arcs were in
action. And in regard to these, we have the observations by
Bubnofl* and Heidenhain,^ and by Exner,* in the narcotized
dog and rabbit In their experiments gentle stimuli to the sldn
of a limb exerted a reinforcing influence on closely following
stimuli applied to the limb region of the cortex of the brain.
Exner s observations proved that minimal electrical stimuli
applied near together in point of time to the fore-limb region
of the rabbit's cortex and to the skin of the crossed foot, ex-
erted a facilitating influence, "bahnung," on each other. He
points out that this reinforcement occurs when the cortex itself

176 COMPOUND REFLEXES [Lect.

has been removed, and the stimulation of the brain is applied
direct to the underlying white matter. He argues, tiierefere,
that the seat of production of the facilitation lies in tiie spinal
centres. With that view, the argument followed here is in com-
plete accord.

The co-ordination, in some of the instances taken, has cov-
ered but one limb or a pair of limbs. But the same principle
extended to the reactions of the great arcs arising in fbe
projicient receptor organs of the head, e. g. the eye, that deal
with wide tracts of musculature as a whole, involves further^
reaching co-ordination. The singleness of action from moment
to moment thus assured is a keystone in the construction of the
individual whose unity it is the specific office of the nervous
system to perfect. And in the instance taken, namely, concur-
rent stimulation of the one fore foot and the crossed hind foot,
the co-ordination can be easily traced further ; the crossed fore
foot is extended at elbow and retracted at shoulder under the
combination of the two stimuli, and the homonymous hind limb
is extended at knee and hip. We might also add to these
movements others, also caused by the same stimulus, of the
eyeballs, the lips, the larynx, and the arterial wall of the
splanchnic area. But these would not for the present purpose
emphasize the main point further.

As remarked above, it is not usual for the organism to be
exposed to the action of only one stimulus at a time. It is
more usual for the organism to be acted on by many stimuli

FiGUkE 40 (opposite). — The skin-fields of the afTcrent spinal roots of the monkey (MacKui
rhesus), showing; their general arraiiv^ement in the trunk and hind limb. On the right side onlf
th*^ potcrior limit of each field is >hown, on the left side only the anterior limit. The fieldis
were (;bscrveri by the method of *- lemaining aesthesia.*' After determination iA tha limits
of the fieli.l for a spinal nxtt in a number of individuals the mean of the observatioos for that
root was tiansfcrrcd to a plaster ca^t of Macacus rhesus, and the lines thus gradually built
up on the mitdel. The dotted line extending from the mid-dorsum out^ntfd along the
dr>rsal aspect of the thigh is the ** dorsal line '* of the hind limb, and to it the fields of
the sensory roots di^tributcd to the skin of the limb behave as do those of the skiii-6elds of
the trunk to the mid-donval line of the body. It will be noticed that the boundaries of the
spinal root-fieMs neither in the limb nor in the trunk conform with the limits of the "re-
ceptive-fiekK " of cut.ine«>us reflexes. The cutaneous fieldit of the "scratch-reflex,** the
" flcxion-retlex." the ** extcnsor-thnsjit." are areas which in nowise fit in with the pattern
of the cutaneous fields of the afferent spinal roots. Compare this figure with Figure 13 A,
Lecture II, p. 46.

178 COMPOUND REFLEXES [Lect.

concurrently, and to be driven reflexly by some group of stimuli
which is at any particular moment prepotent in action on it
Such a group often consists of some one pre-eminent stimulus
with others of harmonious relation reinforcing it, forming with
it a constellation of stimuli, that, in succession of time, will
give way to another constellation which will in its turn become
prepotent.

The concurrent stimuli keep a number of arcs in active
touch with their final paths, and a number of other arcs out of
active touch with the final paths belonging to them. In the
particular instance taken, they keep arcs of one fore limb and
one hind limb in action upon final common paths of flexion of
those limbs, upon final common paths of extension in the diago-
nal pair of limbs, and upon final common paths of flexion of the
neck. And the concurrent stimuli simultaneously check other
arcs from getting into active touch with the final common paths
of extension of one fore limb and hind limb, namely, those of
the seat of stimulations, and of flexion in the opposite fore limb
and hind limb, and of retraction of the neck. Further, these
reactions certainly receive reinforcement through the arcs of re-
ceptors in the muscles and through arcs arising in the receptors
of the otic labyrinth. An instance of reinforcement of this
very kind from muscular receptors we have already given
(Lecture IV, p. 131).

Thus at any single phase of the creature's reaction, a simul-
taneous combination of reflexes is in existence. In this combi-
nation the positive element, namely, the final common paths
(motor neurone groups) in active discharge, exhibits a harmo-
nious discharge directed by the dominant reflex-arc, and rein-
forced by a number of arcs in alliance with it. The dominant
reflex-arc in the instance taken is that from the noci-ceptors of the
right hind foot. The reinforcing arcs are at this phase of the re-
action certain direct extension arcs, certain proprio-ceptive arcs,
and certain labyrinthine arcs. But there is also a negative ele-
ment in this simultaneous combination of reflexes. The reflex
not only takes possession of certain final common paths and dis-
charge nervous impulses down them, but it takes possession of

V] SIMULTANEOUS COMBINATION 179

the final common path whose muscles would oppose those into
which it is discharging impulses, and checks their nervous dis-
charge responsive to other reflexes. This negative part of
the field of influence of the reflex is more difficult to see» but
it is as important as the positive, to which it is indeed comple-
mental. Therefore it is that the reflex initiated by one group of
receptors while in progress excludes in various directions the re-
flexes of other receptors, although these latter may be being
stimulated. In this way the motor paths at any moment accord
in a united pattern for harmonious synergy, co-operating for one
eflfect '

The notion, therefore, that we arrive at of such a motor
reflex reaction is that it is referable to a constellation of con-
gruous stimuli of which one is prepotent, and that the reac-
tion taken in its totality gives the nervous intercommunications
of the central organ a certain pattern, which pattern may
ramify through a great extent of the central organ. This reac-
tion has its positive side traceable as active discharge from a
number of end-points of the nervous network, and its negative
side symmetrically opposed to its positive and traceable con-
versely by check, depression, or absence of nervous discharge.
Even in extensive reflexes of the bulbo-spinal animal it is prob-
able that though great fields of the nervous centres are involved
in the reaction at any one time, large parts are still left outside
the reaction. This part of the neural network would therefore
be indifferent to that particular reaction. That amounts to say-
ing that it is open during the reaction to be thrown into activity
by some concurrent and distinct other reaction. But this
possible neutrality and discreteness of reflex reaction and its
fields is probably far less in the intact higher vertebrate than in
the lower or in the mutilated higher vertebrate.** In the pres-
ence of the brain the knitting together of the whole nervous
network is probably much greater than in its absence.

A question arises concerning the simultaneous combination
of reflexes which is closely related to that regarding the grad-
ing of intensity of a reflex.

Some reflexes exhibit many grades of intensity under grad-

i8o COMPOUND REFLEXES [Lect.

ing of intensity of stimulus. The flexion-reflex is an instance.
There as the skin stimulus is increased the height to which
the foot is flexed is increased. But it seems obvious that
such an effect is not to be expected in all reflexes. Where,
as, for instance, in the scratch-reflex» the foot has, in response
to irritation at a certain spot, to be moved to that spot, it
would defeat the use of the reflex for a strong stimulus to flex
the limb further, so as to carry it beyond the spot required.
And we see that as the scratch-reflex is increased in intensity
the increase does not appreciably increase the amount of tonic
flexion exhibited by the reflex, but spends itself in increasing
the clonic beat of the reflex, which still oscillates about the
same median position. When the scratch-reflex is elicited by
simultaneous combination of two reflexes initiated from spots
near together in the receptive fleld the tonic flexion under-
lying either reflex does not in my experience appear to sum
with that of the other; the summation that appears seems
confined to the more vigorous clonic beating of the combined
reflex.

It seems therefore likely that in the simultaneous combina-
tion of reflexes the reinforcement that goes on, although it is
sometimes expressed as greater amplitude of contraction, is not
necessarily so expressed in all cases. Just as various type-
reflexes exhibit extreme individuality of time-relations, intensity
grading, etc., so also they exhibit in their mode of simultaneous
combination individual differences of high degree.

Vq SUCCESSIVE COMBINATION i8i

LECTURE VI

COMPOUND REFLEXES: SUCCESSIVE COMBINATION.

Argument: Co-ordination of reflex sequences. Chain-reflexes (Loeb).
Overlapping of successive stimuli in time. The sequence of allied
reflexes. Spread of b<ihnungy ** immediate induction." Sequence of
antagonistic reflexes. The r61e of inhibition in this transition. Views
of the nature of inhibition : Rosenthal, Wundt, E. Hering, Gaskell,
Verwom, J. S. Macdonald. The " interference " of reflexes. " Alter-
nating reflexes." W. Macdougall's view of ''drainage of energy.''
*' Compensatory reflexes." Factors determining the issue of the
competition between antagonistic reflexes. '' Successive induction."
Rebound-effects in spinal reactions; tend to restore reflex equilibrium.
Fatigue in reflexes. Relative high resistance to fetigue possessed
by the final common path, /. e. motor neurone. Intensity of reac-
tion a decisive factor in the competition of afferent arcs for posses-
sion of the final common path. Noci-ceptive nerves. Prepotency
of reflexes generated by receptors that considered as sense organs
initiate sensations with strong affective tone. Resistance of tonic
reflexes to fatigue. All these factors render the conductive pattern
of the central nervous system mutable between certain limits.

We considered last the coordination of reflexes in simultane-
ous combination. We now turn to sequence of reflexes. Re-
flexes are seen to follow one another in consecutive combination.
And in this chaining together of successive reflexes in differ-
ent instances different kinds of processes seem traceable. Of
these, one consists in the reaction to one external stimulus
bringing about an application of an external stimulus for a
second reflex. The dart-reflex of the frog's tongue provoked
by the seen fly provides, if successful, the stimulus (contact
with the mucosa of the mouth) which provokes closure of
the mouth, and this probably insures the stimulus for the
ensuing deglutition, and so on. Exner has dealt with this kind
of chaining together of reflexes by one stimulus leading to
another in his " Entwurf einer physiologischen Erklarung psy-
chischer Erscheinungen." Loeb has illustrated it luminously in

i82 REFLEX SEQUENCE [Lect.

his '* Phjrsiology of the Brain." He calls sequence of reflexes
proceeding by this process from one segmental reflex to another
" chain-reflexes " (Ketten-reflexe). Mosso,* Kronecker and
Meltzer, *' *^' ^^^ Chauveau,^^ and Zwaardemaker, ^ ''^ have
traced such reflexes analytically in deglutition.

Mosso ^ showed that in the oesophageal stage of deglutition
each reflex in a part of the tube above is immediately succeeded
by a reflex ensuing in the adjoining part below, and yet in this
sequence the distant bulbar centre is itself concerned, since the
sequence ceases if the branches of the vagus containing the
nerve paths to and from that centre be severed. Biedermana
has recently demonstrated a very similar procession of reflexes
in the crawling of the earthworm, and there again the sequence
involves reflexes conducted through the central nervous system.
It appears that the action of each preceding segment provides
a stimulus for the reflex act of the next succeeding segment.

Orderly sequence of movement characterizes the outward
behaviour of animals. Not least so where, as in the earthworm
crawling,^' or the insect in flight, or the fish swimming, every
observer admits the coadjustment is essentially reflex. One act
succeeds another without confusion. Yet, tracing this sequence
to its external causes, we recognize that the usual thing in
nature is not for one exciting stimulus to begin immediately
after another ceases, but for an array of environmental agents
acting concurrently on the animal at any moment to exhibit cor-
relative change in regard to it, so that one or other group of
them becomes — generally by increase in intensity — tempo-
rarily prepotent. Thus there dominates now this group, now
that group, in turn. It may happen that one stimulus ceases
coincidently as another begins, but as a rule the stimuli over-
lap one another in regard to time. Thus each reflex in the
unmutilated animal breaks in upon a condition of relative
equilibrium ; and this latter is itself reflex.

It was shown that reflex movements can be grouped as
regards their mutual relation into those which have allied rela-
tion and mutually facilitate and reinforce, and those which are
mutually antagonistic. Antagonistic reflexes do not enter into

VI] SEQUENCE OF ALUED REFLEXES 183

simultaneous combination. Simultaneous combination unites
" allied " reflexes only. But into reflex combinations of suc-
cessive kind, reflexes both allied and antagonistic enter as
components.

If the scratch-reflex be excited from a spot in its receptive
skin-fleld and then while the reflex is in progress another
scratch-reflex is excited from a receptive point not far removed
from the first one, the scratch-reflex under the double excitation
may difler very little in appearance from that first excited, and
on the first stimulation being discontinued the reflex persists,
maintained by the second stimulation, and hardly or not per-
ceptibly altered in character fi-om its outset In this case the
second reflex which succeeded the first resembles the first; that
is, it is to outward appearance and for practical purposes merely
a prolongation of it

But if the point of application of the second stimulus
be, although still in the receptive field of the scratch-reflex,
widely distant from that of the first stimulus, the reflex be-
comes obviously modified when the second stimulus is thrown
in. Thus, if the stimulus be so located in the receptive field
that the first excites the low form of the reflex and the second
the high form, the reflex, though initiated in the low form,
assumes the high form when the second stimulus — if that is
of appropriate strength — is thrown in. Or, conversely, it as-
sumes the low form if the second stimulus be appropriately
located for producing that form.

And between the component scratch-reflexes there are grades
of likeness corresponding with degrees of proximity of the points
of application of the stimuli in the receptive field. When, there-
fore, a reflex occurs in immediate sequence to a reflex to which
it is allied, it smoothly maintains the reaction that is already in
progress, and if its own character differs in some respect from
the foregoing reflex, impresses that character on the reflex
without, however, any hitch or hindrance of the reflex.

For a reflex to be immediately succeeded by a reflex of allied
relation to it is of common occurrence. Any stimulus that moves
over a receptive field is likely to excite such a sequence. A

i84 REFLEX SEQUENCE [Lect.

morsel of food moving over the surface of the tongue, a stimulus
moving in the field of vision, an object moving along the skin,
and, as an instance of the last, a parasite .travelling across the
receptive field of the scratch-reflex.*"

In such successive combinations the reflexes are, in the
scratch-reflex at least, linked together by more than the mere
external circumstance of the incidence of the stimulus. In such
a sequence the threshold of each succeeding reflex is lowered by the
excitation Just preceeding its own,

A subliminal stimulus applied at a point A will render a
subliminal stimulus applied at a point B near A supraliminal if
the second stimulus follow within a short time, e. g. 500 <r. The
space of receptive surface across which this can be demonstrated
in the scratch-reflex amounts to 5-6 centimeters. It is best
worked out by unipolar application of the induced current
through a stigmatic electrode — fine gilt entomological pin.
In that way numerical values can be assigned to the results.
But the phenomenon is characteristically and simply illustrated"^
by the difference between the potency as a stimulus of the edge
of a card, say six inches long, pressed simultaneously its whole
length against the receptive skin-field, say for 5 seconds, and
on the other hand lightly drawing one comer of the card along
the same line in the skin-field also for 5 seconds. The former
application simply evokes a reflex of a few beats, which then
dies out The latter evokes a vigorous reflex that continues and
outlasts the application of the stimulus. A successive line is
more effective as stimulus than a simultaneous line of equal
length and duration. Again, if a light disc three centimeters in
diameter and a fraction of a millimeter thick be freely pivoted
in bearings at the end of a handle, so that it turns when pushed
by its handle over the skin surface, such a wheel may not, when
pushed against a spot of the receptive surface, excite the reflex,
but it excites it when it is rolled along it. The same thing is
seen with a spur wheel. Even when the points are two centi-
meters apart, as the spur wheel is rolled over the surface succes-
sive summation occurs, and the reflex is evoked as the progress
of the wheel proceeds. If a parasite in its travel produces ex-

VI] IMMEDIATE INDUCTION 185

citation which is but close below the threshold, its progress is
likely to so develop the excitability of the surface whither it
passes that the scalptor-reflex will be evoked. In the skin
and the parasite respectively we have, no doubt, two compet-
ing adaptations at work. It is perhaps to avoid the conse-
quences of the spatial spread of the ** bahnung " that the hop
of the flea has been developed.

This iHihnuHg^ which spreads around a stimulated point, must
be the same phenomenon which finds still more marked ex-
pression in the summation of stimuli individually subliminal
applied successively at one and the same point That it in-
fluences other points in the neighbourhood as well as its own
seat of application, is another item of evidence of the central
conjunction of the neighbouring reflex-arcs of any one type-reflex.
It is an influence which, with barely supraliminal stimuli, is short
lived ; but it is one of the factors in the card-experiment just
mentioned. This spread of influence to adjacent points is im-
portant because, though short lived, it can contribute effectively
to maintain one and the same reflex. It favours the occurrence
of sequences of closely allied reflexes. It is convenient to have
a term for such a species of '' bahnung^^ and '' immediate induc-
tion " seems the most suitable here.

Phenomena akin to these are met with in the physiology
of vision. A moving point in the peripheral field is more
visible than a line of similar length, direction, and duration.
Again, a row of dots individually below the minimum visibile and
too far apart for their retinal images to overlap by diffusion,
becomes visible. Probably this same process is contributory
toward the seeing of lines the diameter of which is narrower
than the diameter of the circular minimum visibile. I have found
osmic-stained nerve-fibres of 4 fi diameter visible to the naked
eye both for myself and other workers in the laboratory. Re-
inforcement by positive induction appears to be at work in these
visual eflfects just as in the scratch-reflex.

In the sequence of reflexes the supervening reflex may differ
imperceptibly or slightly, though distinctly, fi-om the precedent
or may be in part quite different from that

Ficums jt^ — Effect of overlapping in time ol the stimuli for the flciion-reflcx and the
KSBlcb-refle^t ^^^ *•''-• ^'gnal showi the stimulus for the scratch refleac, the lowo-
^pul that fof t -flex. Time in seconds bdow. The sciatch-reflcx seems

to dispute the tl .- ; had it fused with it the ctirobincd lift of the lever innald

lavt been much higher ibaa it is (spmal doj). Compare the scratcb-reflcjt above in the
figure to left.

If the reflexes are closely similar, it is difficult to say at what
moment the traasition under overlapping stimuli occurs, since
the initial reflex on discontinuance of the first stimulation is

i88 REFLEX SEQUENCE [Lbct.

maintained unaltered by the second. But if the reflexes arc
recognizably dissimilart /. g. a low scratdi-refiex and a high
scratch-reflex, the moment of transition is obvious, for tbe
reflex then takes the form of the response which the second
stimulation would excite, and, on discontinning the first ^oh
ulation, is continued in that form (Fig. 50). In my experiai^
the transition does not, in the case of this reflex at least, in-
clude a period of summation of the two reflexes in the sense
that the reflex under the two stimulations A and B consists of
the response to stimulation A summed with that to stimub-
tion B. So also in the transition from one reflex to another
of even greater dissimilarity. There is the same absence of
any period of fusion of the two reflexes. Such fusion might
be appropriately termed confusion. The rule seems that siicli
confusion is avoided in the transition. In the hind limb a
scratch-reflex of the high form presents a certaiti degree of
resemblance to a simple flexion-reflex, inasmuch as there can
be distinguished in the former a marked tonic flexioa on
which the clonic is, so to say, superposed. When a scratdi*
reflex of this form supervenes on the flexion-reflex the result
is commonly that which is seen in Fig. 51. The tr ansition
occurs without confusion; even in regard to the tonic con-
traction, an element possibly common to the two reflexes,
there is in the transition no period at which the tonic con-
traction of the flexion-reflex has added to it that of the
scratch-reflex. In the instance figured the amplitude of the
tonic contraction of the scratch-reflex was about equal to that
of the flexion-reflex ; if these summed, therefore, their joint am-
plitude would be much greater than that of either reflex singly.
But the record shows a smooth continuity of flexion-reflex with
scratch-reflex in which there is no stage of summated amplitude
of the two contractions. This is what I mean by saying that
the transition from one reflex to another takes place without
confusion.

Not that the onset of one reflex is uninfluenced by the exist-
ence of the other — its interval of latency is, for instance, liable
to be gready influenced. In the example furnishing Fig. 51,

^^^tGURi^ 5a* — ^Th« displicetncnt of the 5tcpping-rcti« by the scratch-reflex during the con-
|H IniMDce of the itimuUtioQ appropriate for the former. The upper ^gntl indicates the
^^ itnottltt* for the soatch-reflex, namely, 75 break shocks dclWered (ttnipoUr faradi cation)
^H ■! mc of 30 per second. The lofter signal give» the stimulus for the stepping-reilex —
^V a Myai »d reflex, tlie stimttlus (uidpolar faradization) being delivered to the opposite foot, .
Tkt soalch-rcfiex, after a considerable latency, displaces the stepping-cellex* The oossei]
steppiiif-refln reappears only in modVined and imperfect form, thoagb its stimului Iti
imaltered for some seven leoovids after the end of the stimulus for the scntch*
Time marked below in seconds.

the scratch-reflex, though its intensity is not increased, shows
a hesitancy about the opening of its clonus not present in the
reflex when elicited singly. Again, in Fig. 52, which shows
transition from the crossed stepping-reflex to the scratch-reflex,
though no period of confusion occurs in the transition from the
, former reflex to the latter, there is yet, on cessation of the latter^

I90 REFLEX SEQUENCE [Lect-

evidence of modification of the former. The crossed stepping-
reflex returns, but considerably modified and after a longer
latency than before. The amount of modification is greater
than would in my experience be ascribable with probability to
the effect of mere fatigue for the period during which the stim-
ulus was at work, although the movement itself was in abeyance;
the rhythm is present but weakened.

If it is advantageous for the transition fi'om one reflex to
another of like type to occur without a period of confusion, it is
still more advantageous that it should be so in the case of tran-
sitions from one reflex to another of converse type. Confusion
in the literal sense above would in that case involve not merely
inaccuracy at the outset of each new reflex, but would mean
mutual destruction of the two reflex effects ; an interval of impo-
tent mutual self-hindrance would disadvantageously intervene
between successive motor acts of opposite direction. In the
transition from one reflex to another of antagonistic kind the
avoidance of confusion of the two reflexes emphasizes at the
same time the impossibility of co-ordinating them in a simul-
taneous combination.

Though the stimulus exciting the reflex that is displaced
continues while the new reflex is introduced, the displacement
of the former reflex occurs without confusion.

Taking the flexion-reflex and the scratch-reflex, the one may
temporarily interrupt the other in mid career (Figs. 43, 53) or
may cut it short or may defer its onset; in all these cases it
does so without a phase of confusion in the transition, although
the stimuli belonging to both reflexes continue in contemporary
operation (Figs. 43, 51, 52, etc.) all the time. And the same
holds between other antagonistic reflexes, e, g. flexion-reflex
and crossed extension-reflex (Figs. 30, 32, 33), extension-reflex
and scratch-reflex (Figs. 42, 54), etc.

And the direction of the interference is reversible. The
flexion-reflex may be made to interrupt the scratch-reflex (Fig.
43) or the scratch-reflex to interrupt the flexion-reflex (Fig. 51).
The scratch-reflex may be made to interrupt the crossed step-
ping-reflex (Fig. 52) or the crossed stepping-reflex to interrupt

VI]

INHIBITION

FiGumx 53. — A- Scraich-rctiex uitcrrtjpted by a br.ti "—"•^pri:. The time of appticsu

tion of the sttmulus evoking scratch-reHex is shuwi st signal Uae; that of the

stimulus of the Eexion-re^cx in the signal !iae ui. y above the other* Time

marked in fifths of seconds at top of the ccoord. The icntch-refles returns with in-
creased intecKitr after the interruptino.

6. Siinilir to A, but the »cratch*reflex is interrupted later and returns more ikmly and
with ottrked irre^uUrity in its beat.

the scTatch-rcflex (Fig. 55); and similarly with other pairs of
antagonistic reflexes.

Inhibition. The process in virtue of which this transition
from one antagonistic reflex to another occurs is obviously one
of active intervention. In many cases the form which the in-
tcr\xntion takes is inhibition. For instance, where a crossed
extension-reflex is interrupted by a brief flexion-reflex, two
transitions occur, the first at a moment a from the E-reflex to
the F-reflex, the second at a moment S from the F-reflex back
to the E-rcflex (Fig. 33, p. lOl ). Although stimuli for the two
reflexes are both in operation continuously from a to ^, there
IS a clean transition from one reflex to the other. The tracing
is taken from the extensor muscle of the knee. The flexion-
reflex expresses itself by inhibition of the reflex contraction in
progress in that muscle at that time under the combined in-
fluence of the crossed extension-reflex and of the tonic extensor

REFLEX SEQUENCE

PLect.

FiGtTRB 54. Tht scratch-reflex cut short by excitation of the sldn of a tUgit of the 1

bind foot, Below, tlie upper signal marks the period of application of the stiiniiitis to
the opposite hind foot, the lower iiga^\ marks the period of application of the 1
exdtine the scntch^rdkn. Time above io ^ leDOodft.

tittanli^i

rigidity due ta the animal being decerebrate. It causes the
contraction due to these conjoined reflexes to cease. In such
a case the transition from the extension*rcflex to the flexion-
reflex evidently occurs by inhibition. So also in the transition
from the flexion-reflex to the extension-reflex when the ham-
string muscles are examined (Fig. 32, p. 98).

We do not yet understand the intimate nature of inhibition*
In the cases before us now. its seat is certainly centra^ and in all
probability is, as argued above, situated at points of synapsis.
I have urged that a prominent physiological feature of the
synapse is a synaptic membrane. It seems therefore to me
that inhibition in such cases as those before us is probably
referable to a change in the condition of the synaptic membrane
causing a block in conduction. But what the intimate nature
of the inhibitory change may be we do not know.

The views of some of those who have authoritatively ti«mted

VI] INHIBITION 193

of the nature of inhibition may be cited here, both for their in-
trinsic interest and the suggestion of lines of investigation. One
view has been that, as the process of conduction along nerve-
fibres is an undulatory one in the sense that the nerve-impulse
travels as a disturbance with wave-like configuration of inten-
sity, inhibition is due to a mutual suppression of two wave-like
disturbances impinging on the same point of the conductor but
in opposite phases of disturbance.

In those cases where stimulation through one nerve inhibits
the action of a tissue acting under another nerve we can imagine
a process which leaves the tissue unaffected but simply inter-
feres with another stimulus, as in the physical interference of
vibrations. Rosenthal's ^ resistance theory of the action of the
afferent vagus-fibres upon the respiratory centre is a supposition
of this kind. It recognizes during the inhibition no change of
total output of energy in the particular function inhibited.
The alteration of a hypothetical resistance only distributes the
discharge of the nerve-centre over an altered time-rhythm, —
smaller and more frequent discharges representing the same lib-
eration of energy as larger and less frequent. Such a view of the
nature of inhibition is that which Gaskell ^^ termed the " neutrar*
one ; according to it the inhibition leaves the tissue in the same
ultimate condition as that in which it found it, neither exhausted
nor surcharged. " As far as the central nervous system is con-
cerned, there exists a strong general tendency to look upon
the inhibitory processes occurring there as * neutraV in their
character. " i«

This view, originally put forward by v. Cyon,* has been
abandoned by him, although it has since been supported by
Lauder Brunton ^^' and others. It was expressly dissented from
by Wundt.^ And the grounds of Wundt's objection are valid,
the kernel of them being that though in a certain sense of the
word the nerve-impulse can be described as wave-like it is not
an undulatory disturbance at all in the sense in which those
reactions arc which show physical interference. It is therefore
to physical interference that in this view of inhibition the
analogy is drawn, but the similarity between the process of

REFLEX SEQUENCE

FtGtTitK $;. — Tbe scmtcb-redex Interrupted by the crossed stepping-ctfleE. The
signal of the two beloi* the myograph trace shows the period of application of the
Utioti (unipoUr fafadixatioa of shoulder sldo) evtildog tbe »cratcb^reflex. During the
contiouance of this sUmubtiQfi m fairly strong stimiilatioD of the oossed foot was applied,
as marked by the lower lignal at bottom of the reoofd. The inhibitioQ outlasts the
application of thit second stimalus by some 3 secxmds ; the saatch-reDex then returm,
and only ceases on CPiwtlnn of its own stimulus. Time marked m fifths of
abovcL

nerve-conduction and conduction of light and electrical waves
or sound» etc., is not real enough to strictly justify such analogy.
Moreover, as we shall see presently, central inhibition is not a
neutral process* for, at least in many cases, it leaves the reflex
centre surcharged for subsequent response (f. infra^ pp, 205-213,
*' successive induction ").

The most striking thing that we know of inhibition is that
is a phenomenon in which an agent such as in other cases excites!
or increases an action going on in this case stops or diminishes
an action going on. Now, the activity of a tissue can be low-
ered or abolished by production in it of deleterious changes
such as exhaustion or, in the highest degree, death. But there
is no evidence that inhibition of a tissue is ever accompanied by
the slightest damage to the tissue; on the contrary, it seems to
predispose the tissue to a greater functional activity thereafter

is"

VI] INHIBITION 195

We can imagine that a material continuously produced by
a tissue, and yielding on decomposition the particular activity
which is inhibited, may by an inhibition be checked in its
decomposition, and accumulate, so that at the end of the period
of inhibition the tissue contains more of the particular decom-
posable material than before. This molecular rearrangement
would diminish activity for the time being, but lead to increase
of activity afterwards. There would ensue a rebound effect.
This is, as is well known since Gaskell's*^ researches, what
actually happens in the pure vagus action on the heart. A
similar rebound-effect is perfectly obvious in many instances
of inhibition in the central nervous system.** It constitutes a
point of resemblance between central and peripheral inhibition.

Such an explanation as this second one may take the chem-
ical structure of the living material, the bioplasm^ of the cell as
the field of operation for the decomposition and the synthetic
process that it pictures. The living cell is constantly liberating
energy in its function, and rebuilding its complex structure
from nutrient material. Its life is therefore an e;quilibrium of
balanced katabolism and anabolism ; at any given moment the
one process or the other may predominate in the cell. At a
moment when the cell is vigorously discharging some function
which involves conversion of internal energy into external energy
it is in a katabolic phase. In subsequent relative repose from
the discharge of that function the replenishment of its store of
potential energy by assimilation may predominate and the cell
be in an anabolic phase. Katabolism and activity of external
function, anabolism and rest from external function, come on
this view to be almost synonymous terms. But Hering, Gaskell,
Verwom, and others have taught us to attach important external
functions to the assimilatory (anabolic) phase. The first men-
tioned has dealt with the visual sensations in assimilation-dis-
similation pairs. Black-white sensation is thus traced to a pair
of reactions affecting the trophism of the cell in exactly opposite
directions. Gaskell relates vagus inhibition of the heart to the
throwing of the cardiac muscle-cell into a phase predominantly
anabolic. Verwom in his " Biogen-hypothese " has developed

196 RFFLEX SEQUENCE [Lect.

this trophic theory with further elaboration still. These views
all take the actual nutritive activity of the cell as the direct and
immediate field in which inhibition has its seat The tendency
to rebound after-eflfect in the opposite direction is in this view
a natural trophic result. Hence Gaskell expressively speaks of
the vagus as the " trophic nerve of the heart." And Hering^
(1872)^ formulated his experience somewhat as follows:
The action of a stimulus affects the cell's autonomic trophic
equilibrium ; it may increase or lessen either the cell's asssimi-
lation or the cell's dissimilation ; in whichever of these ways the
stimulus acts the excitation of the cell, owing to a self-regulation
proper to it, dwindles for that stimulus and for all stimuli produc-
ing a similar change, while the excitability increases for stimuli
that produce an opposite effect. On all these views inhibition
is in its intimate and essential nature a trophic process.

An hypothesis not built immediately on views of the nutri-
tive processes of the cell has recently been put forward by Mac-
donald.^^ Dealing with nerve and muscle fibres, he does not
take the purely chemical structure of the living framework of
the cell as the field of operation for either excitation or inhibi-
tion. The explanation he offers of these two latter processes
is as follows.

From study of the part played by inorganic salts in the
function of nerve, he sees in the attachment of these salts to
the proteids present, and in their partial detachment from the
proteids, normal occurrences underlying the conditions of rest
and excitation respectively. He assumes the connection be-
tween salt and proteid involved in this matter to be of purely
physical nature. The axis-cylinder, for example, is composed
of a colloid solution in which there are present minute particles
of colloid proteid. These may increase in size — indeed so
much as finally to become visible — under the influence of
factors determining a tendency towards coagulation. Upon the
surface of these particles the major portion of the inorganic salts
present is held restrained in a condition of condensed solution.
An increase in the size of the particles is accompanied by a
diminution in the total surface separating the particles from the

VI] INHIBITION 197

solvent in which they lie. Such a diminution in surface is
equivalent to a diminution in the forces restraining the motion
of the inorganic salts. It occasions the liberation of salt mole-
cules in a state of free motion into the surrounding aqueous
solution. This release of hitherto restrained molecules is the
cause of alterations in osmotic pressure, of new processes of
diffusion, of resultant electrical phenomena, and thus of the
phenomena of excitation.

Macdonald thus considers a stimulus as an agency determin-
ing an approach to the condition of coagulation. He regards
as the important characteristic of the excited state the release
of inorganic salts resulting from this change. According to him
inhibition is a condition in which the inorganic salts are more
securely packed away upon the surface of the '* colloid particles "
than usual by reason of diminution in the individual size of the
particles and an increase in the surface they present to the sur-
rounding solution. He supposes the communication of a nega-
tive electrical charge to be a stimulus provoking a tendency to
coagulation with all the just-mentioned dynamic consequences
of the enlargement of the colloid particles. So he considers
that conversely the communication of a positive charge produces
a change of an exactly opposite kind in which inorganic salts
hitherto in motion are brought to a static condition of rest

It does not seem at once clear that the condition of greater
subdivision of particles should also be a condition of greater
stability, although it is clear that such a conception might ex-
plain the greater store of potential energy possessed by an
inhibited tissue. Macdonald, in fact, does not, I take it, assume
this to be the case. To explain this separate fact he appeals to
the evidence upon which the conception is based and points out
a distinction between the amount of inorganic salt involved in
changes above (inhibition) and changes below (excitation) the
equilibrium line of the normal resting state. Thus there is
quantitative evidence to show that the amount of salt remaining
for withdrawal from the resting colloid solution is only a small
fraction of the total amount of salt present ; the major portion
is already withdrawn, and is, so to speak, in reserve in this large

198 REFLEX SEQUENCE [Lect.

quantity for such changes of excitation as occur below the base
line of rest Changes from the normal to the hypemormal, and
from the hypemormal to the normal, cannot therefore involve a
redistribution of salt comparable in quantity to those taking
place down from, and up to, the normal. It is in this way con-
ceivable that the application of a stimulus to an inhibited tissue
— although productive of an effect akin to the phenomenon un-
derlying the process of excitation — might yet lead only to such
a subminimal change in the amount of inorganic salt in motion
as to determine no externally appreciable manifestation of its
occurrence. Imagine a tissue which has been placed in a con-
dition of inhibition by the communication of a positive electrical
charge. The application of a negative charge to such a tissue
would produce no visible effect, although productive of an in-
ternal change. The application of a second negative charge
would give rise to a characteristic excitation, its efHciency deter-
mined by " summation." Let us on the other hand suppose
that the tissue has not only been inhibited, but is maintained in
a continuous state of inhibition by the steady arrival of positive
charges. In this case the application of a succession of stimuli
would result in nothing more than a series of subminimal, and
therefore unnoticed, changes.

In an excited tissue, summarizing this conception, an unusual
quantity of inorganic salts is in motion. Excitation is ended by
the reduction of this excessive motion. Inhibition is the condi-
tion in which the possibilities of free motion are most reduced.

This view is fertile in suggestion for further experiment.
Based on examination of the physical structure of nerve by
electrical methods irreproachably employed, and on the revela-
tion, under the microchemical test which we owe to Macallum,*^*
of potassium appearing in quantity at injured points of nerve-
fibres, and explaining naturally as it does the injury-current of
nerve as similar in production to the current of a " concentra-
tion" battery ^^ the concentrations of which can be known from
the current, it merits very careful consideration. The features
and conditions of occurrence of inhibition harmonize strikingly
with what on Macdonald's view we should expect them to be.

VI] INTERFERENCE 199

Intoifereiioa. Whatever the intimate nature of the inhibition,
it is, however, only one part of the processes involved in transi-
tion from one antagonistic reflex to another. In the transition
from the crossed extension-reflex to the flexion-reflex the in-
hibition of previous excitation in the extensor neurone is accom-
panied by excitation of the previously inhibited flexor neurone.
And conversely in the transition from flexion-reflex to extension-
reflex. Transition from one form of reciprocal innervation to
another will obviously involve such changes wherever the transi-
tion is from one reflex of simultaneous double-sign to an antago-
nistic of simultaneous double-sign. There will be inhibition at
one set of points and excitation at another. The process of
transition, therefore, in many cases is one half of it inhibitory,
one half excitatory. It seems advisable, therefore, to avoid em-
ploying the term ''inhibition" for the displacement in general of
one reflex by another. To avoid confusion, some expression of
broader scope, including excitation as well as inhibition proper,
seems required. The term "interference" already used by
Wundt ® and by A. Tschermak ^ in an almost similar way seems
to me well suited for this purpose. In employing it Wundt
expressly stipulated that it had in this use no reference to its
technical employment by physicists for the mutual interaction
of vibrations, as in light and sound. The term " interference "
as applied to reflexes would mean simply the interaction between
antagonistic reflexes, that is, reflexes which are incapable of
simultaneous combination. These reflexes are capable of suc-
cessive combination, and in that process the influence of one
reflex replaces that of another upon a common path potentially
belonging to each. The replacement may take place by inhi-
bition succeeding excitation, or by excitation succeeding in-
hibition, or by excitation of one kind succeeding excitation of
another kind, as when the steady tonic flexion of the flexion-
reflex is succeeded by the rhythmic clonic flexion of the
scratch-reflex. In all these cases the process of displacement
of one reflex action by the other may be termed interference.
We thus get a comprehensive convenient term for embracing
the whole series of cases.

200 REFLEX SEQUENCE [Lect.

The frequency with which, in co-ordination of reflexes by
successive combination, the reflex which succeeds to another is
antagonistic to this latter is very great. Two important classes
of such sequence are especially common. One is that which
forms what are known as " alternating reflexes ; " the other is
the class of " compensatory reflexes."

Alternating reflexes are seen very clearly in cyclic reversals
of direction of movement; thus, when extension succeeds
flexion in the stepping-reflex. Here antagonistic reflexes suc-
ceed one another alternately at two flnal common paths. In
the motor neurones for the knee, in the stepping-reflex, excita-
tion and inhibition alternately ensue in the flexor neurones, while
synchronously with that, inhibition and excitation alternately
ensue in the extensor neurones.

The essence of an alternating reflex is that excitation and
inhibition ensue in succession at two (or more) flnal common
paths, a sequence of antagonistic reflexes possessing them in
turn. In an ordinary rhythmic reflex a periodic excitation
(and a periodic refractory or inhibitory state) is recurrently
produced in the reflex-arc at rhythmic intervals. Every alter-
nating reflex, therefore, is a rhythmic reflex, but not every
rhythmic reflex is an alternating reflex. The movements of the
vertebrate limb in locomotion give instances of alternating re-
flexes ; probably the movement of the tail of fishes in swimming
is a similar instance, but has not yet been analyzed as to its reflex
composition. Alternating reflexes form an excellent field for
examination of reciprocal innervation of antagonistic muscles.

It is particularly by the case of " alternating reflexes " that
Macdougall illustrates his view of the central process in re-
ciprocal innervation. His scheme also offers an explanation
for the transition from one antagonistic reflex to another.
Based more immediately on the behaviour of visual images,
it is applied by Macdougall specifically to the case of the
reciprocal innervation of antagonistic muscles. "Let us." he
writes,2«2 "imagine each arc in a simple schematic form as a
chain of three neurones afferent (^j), central (^Zj), efferent
(tfj), and let us call them a^, a^, and a^, and b^, b^, and b^, in

VI] ALTERNATING REFLEXES 201

Figure 56.— Explanation in text.

the two arcs respectively" (Fig. 55). "When a strong stim-
ulus is applied to the afferent neurone of arc a it generates
neurin rapidly, so that it becomes very rapidly charged, and the
resistance of synapse a-^^ — a^ is lowered until a series of dis-
charges takes place from a-^^ to a,, and again from a^ to a^. The
problem is, then, to imagine such a mode of connection between
arc a and arc b as will cause arc a during stimulation to drain off
from the afferent and central neurones of b the smaller quantities
of neurin generated in them. Several forms of such a connection
may be imagined, but I think that probably it takes the form of
a collateral fibre coming from neurone d,, and taking part with
the axone of a^ in forming a synapse with ^3." " Whatever the
exact constitution of this synapse may be, we may assume that,
when its resistance is lowered by the stimulation of a, and con-
sequent charging of a^, the collateral of b^^ making connection
with 03, through this synapse, becomes the path of least resist-
ance for the escape of neurin from b^ and b^. These neurones
are therefore drained by a^, while b^ ceases to receive any neurin
from ^2, and the tone of the muscle-group supplied by it is
abolished." " In a similar way, if both a-^ and b-^ be stimulated,
but one more strongly than the other, the more strongly stimu-
lated arc will drain the afferent and central neurones of the less
strongly stimulated arc, because the resistance of synapses of
the former will be reduced to a lower level than that of the
synapses of the latter."

This scheme fits a number of facts of reciprocal inhibition.

202 REFLEX SEQUENCE [Lect.

Thus, in reciprocal innervation, as the term itself implies, the
inhibition at one part always appears as the negative aspect of
positive excitation at another. To *' alternating " reflexes, which
are common as spinal reactions, Macdougall applies the scheme
thus : " We must suppose the collateral connection of the arc a
(to which we may suppose the stimulus to be applied) with the
neurone b^ to be but little inferior in conducting capacity to
its direct connection with a^. When, then, the neurone a is
stimulated continuously, the arc will first discharge into a^ and
drain b^ and b^ (i. e. inhibit ^3) until after some little time fatigue
causes the resistance of synapse a^ — a^ to become slightly
greater than that of synapse b^ — dg, when a^ will discharge
with b^ wholly into b^, and a^ will be in turn " inhibited/' So
the charges of the aflferent neurones of both arcs will be dis-
charged into ^3 until fatigue causes rise of resistance of the
synapse on 63." The high influence of intensity of reaction
in determining whether the reaction shall or shall not replace
another reaction is expressed by this scheme very lucidly. Also
the occurrence under strychnine*^ and tetanus toxin of excita-
tion at a synapse where inhibition is otherwise the rule seems
a contingency which the view answers well. If these agents
(strychnine presumably from the afferent side, tetanus toxin
from the efferent) reduce the resistance at synapse a^ — b^, there
will, on stimulation ol a^, be conversion of the inhibition of b^ in
excitation, just as in the second phase of an alternating reflex,
but the resistance at synapse a^ — a^ not being raised, as it is
in the second phase of the alternating reflex, a^ will also be
excited as usual, and both antagonistic muscles will contract, as
I have shown they do in fact in strychnine reflexes and as they
do in the convulsions of strychnine poisoning. The scheme
makes it clear, too, that this double discharge, or leakage, will
prove rapidly exhausting on the central arcs, and indeed the
phasic character of the attacks in strychnine poisoning does seem
due to rapid exhaustion following each convulsive discharge.

Again, in working with tetanus toxin, I have,*^ in the
gradual progress of the disease, several times found the afferent
nerves produce a slight reflex inhibition of the extensor of the

VI] COMPENSATING REFLEXES 203

knee, if the initial posture at the knee were at the time exten-
sion (i. f., the extensor arc in high activity), and yet produce
distinct excitation of the extensor if the initial posture at the
time were flexion (/. e., the extensor arc in less activity). These
effects of strychnine and tetanus toxin the view of Macdougall
seems well fitted to meet, though they were not known at the
time the view was formulated. The view seems also applicable
to some of the results obtained in the interesting experiments
by V. Uexkiill on Invertebrata.

One difficulty however seems to me presented by Macdougall's
view, and it is that the diversion of the influence of a^ through
a^ away from a^ which constitutes inhibition does not suggest
a reason for the superactivity (successive induction) in a conse-
quent on the inhibition. And a more serious difficulty, attaches,
in my thinking, to the view, — at least, as it at present stands —
in that it seems to sever this central nervous inhibition — of
which I regard reciprocal innervation of antagonistic muscles as
but one widely spread case — from other forms of inhibition
met peripherally in the heart, blood-vessels, and viscera, rather
than to connect it with them. It appears to me unlikely that in
their essential nature all forms of inhibition can be anything but
one and the same process.

Another important class of sequence of antagonistic reflexes
is that which gives the " compensatory reflex." A compensa-
tory reflex occurs where the reflex is a return to a state of reflex
equilibrium which had been disturbed by an intercurrent reflex
to which the compensatory reflex is the diametrical antagonist
Some compensatory reflexes are excited by passive movements,
others by active movements. It is the latter which come under
consideration here. Many reflex movements are intercurrent
reactions, breaking in on a condition of neural equilibrium itself
reflex. Take the case of the flexion-reflex of the leg (spinal dog)
induced by a brief stimulus during " decerebrate rigidity," where
the animal may be regarded as a bulbo-spinal machine. Sup-
pose the animal suspended with spine horizontal and limbs
pendent The limbs are then in slight active extension, much

204 REFLEX SEQUENCE [Lect.

as if the animal were standing. This slight extension is active,
for it is reflex, and the peripheral source of the maintained re-
flex pose is traceable to arcs arising in the extensor muscles, in
alliance probably with some from the otic labyrinth. The crea-
ture breathes quietly and regularly. Its skeletal musculature
otherwise exhibits no movements, although its reflex activity is
considerable and in progress all the time, as shown by the steady
reflex extension of the limbs. If, then, the foot be excited by
a brief stimulus and thus a flexion-reflex induced in the limb,
the limb is drawn up at hip, knee, and ankle. The movement
is brief; after being drawn up, the limb returns to its previous
pendent posture. It is easy in many instances to perceive that
the pose of extension, as resumed, is more marked than it was
prior to the intercurrent reflex or flexion. It is also equally
easy to perceive that in the replacement of the limb the exten-
sion is not a mere passive drop under gravity, but is a rever-
sion to the previous posture by an active movement In such
a case the reflex effect of the intercurrent stimulus seems to
cease with the cessation of the intercurrent reflex (flexion)
which the stimulus immediately provoked. But closer examina-
tion shows that this is not really the case. There is an active
reflex return to the pre-existing pose. Thus, the disturbing
stimulus brought about not only the flexion-reflex, but, second-
arily to that, a reflex antagonistic to that This latter antago-
nistic (extension) reflex is " allied " to that which originally held
the field. When the flexion-reflex disturbed the neural equi-
librium it dispossessed the opposed reflex of extension from
certain final paths common to that and itself. In other words,
its own reaction induces an after-coming reflex antagonistic to
itself, and this brings resumption of the original reflex attitude
that, under the condition (gravity, etc.) obtaining at the time,
satisfies neural equilibrium. The whole intercurrent reflex dis-
turbance is really ended by a "compensatory reflex." The
compensatory reflex in this case seems traceable primarily to
proprio-ceptive (muscular) afferents from the muscles, joints,
etc., of the limb. But compensatory reflexes are particularly
evident in reflex reactions started by labyrinthine afferents

VI] FACTORS DETERMINING THE SEQUENCE 205

(Ewald, Lee, Loeb, Lyon, Muskens, Nagel, and others), and in
the decerebrate animal this compensatory reflex of the limb
is presumably due to muscular aflferents of the limbs and to
labyrinthine afferents acting in reflex alliance.

Among the aflerent nerves of importance to this compensat-
ing reflex there seems particularly that of the vasto-crureus muscle
itself, the extensor of the knee. Electrical stimulation of the
central end of that nerve excites contraction of the flexors of
hip and knee and inhibits the vasto-crureus itself. It therefore
reinforces the flexion-reflex, but its stimulation is, on being dis-
continued, immediately succeeded by contraction of the extensor
muscles. This rebound (successive induction) is particularly
marked in the vasto-crureus itself. These instances exempli^r
further what was said as to the close connection of secondary,
not immediate, kind between reflexes initiated by receptors of
the extero-ceptive {e^g. skin) surface and reflexes initiated by
receptors of the deep, 1. e. proprio-ceptive field. But in the
instances given earlier the proprio-ceptive reflex which was
initiated secondarily in consequence of the foregoing extero-
ceptive (skin) reflex is antagonistic to the latter; the two
reflexes are related as antagonistic reflexes. The secondary
association which, as pointed out earlier, holds so generally be-
tween certain extero-ceptive proprio-ceptive pairs of reflexes
and connects them, forms some of its pairs from " allied re-
flexes " and others from " antagonistic reflexes." In the former
case the coupling is " simultaneous," in the latter " successive."
Proprio-ceptive reflexes may themselves be coupled in antago-
nistic pairs ; of this, one example is the reflex contraction of the
hamstring muscles, which sometimes undoubtedly ensues on stim-
ulation of the central end of the nerve of the extensor of the knee
(I was at first inclined to attribute this to escape of current, but
am convinced it occurs truly reflexly sometimes) ; and another
example is Mislawski's and Baglioni's interesting expiratory
reflex elicitable from the afferent fibres of the phrenic nerve.

Fkoton determining the sequence. The formation of a com-
mon path from tributary converging afferent arcs is important

206 REFLEX SEQUENCE [Lect.

because it gives a co-ordinating mechanism. There the domi-
nant action of one afferent arc, or set of allied arcs in condomin-
ium, is subject to supercession by another afferent arc or set of
allied arcs, and the supercession normally occurs without inter-
current confusion. Whatever be the nature of the ph3fsiological
process occurring between the competing reflexes for dominance
over the common path, the issue of their competition, namely,
the determination of which one of the competing arcs shall for
the time being reign over the common path, is lai^ly condi-
tioned by four factors. These are "spinal induction,"** rela-
tive intensity of stimulus, relative fatigue, and the fiinctional
species of the reflex.

I. Spinal indaction. The first of these occurs in two forms,
one of which has been already considered, namely, " immediate
induction." It is a form of " bahnung." The stimulus which
excites a reflex tends by central spread to facilitate and lower
the threshold for reflexes allied to that which it particularly
excites. A constellation of reflexes thus tends to be formed
which reinforce each other, so that the reflex is supported by
allied accessory reflexes, or if the prepotent stimulus shifts,
allied arcs are by the induction particularly prepared to be
responsive to it or to a similar stimulus.

Immediate induction only occurs between allied reflexes. Its
tendency in the competition between afferent arcs is to fortify
the reflex just established, or, if transition occur, to favour transi-
tion to an allied reflex. Immediate induction seems to obtain
with highest intensity at the outset of a reflex, or at least near
its commencement. It does not appear to persist long.

The other form of spinal induction is what may be termed
successive induction. It is in several ways the reverse of the
preceding.^^

In peripheral inhibition exemplified by the vagus action on
the heart the inhibitory effect is followed by a rebound after-
efiect opposite to the inhibitory (Gaskell). The same thing
is obvious in various instances of the reciprocal inhibition
of the spinal centres. Thus, if the crossed-extension reflex of
the limb of the spinal dog be elicited at regular intervals,

2o8 REFLEX SEQUENCE [Lect.

say once a minute, by a carefully adjusted electrical stimulus
of defined duration and intensity, the resulting reflex move-
ments are repeated each time with much constancy of char-
acter, amplitude, and duration. If in one of the inten^als
a strong prolonged {e. g. 30") flexion-reflex is induced from
the limb yielding the extensor-reflex movement, the latter
reflex is found intensified after the intercurrent flexion-reflex.**'
The intercalated flexion-reflex lowers the threshold for the
aftercoming extension-reflexes, and especially increases their
after-discharge (Figs. 57, 58). This effect may endure, pro-
gressively diminishing, through four or five minutes, as tei^ted
by the extensor-reflexes at successive intervals. Now, as we
have seen, during the flexion-reflex the extensor arcs were
inhibited: after the flexion-reflex these arcs are in this case
evidently in a phase of exalted excitability. The phenomenon
presents obvious analogy to visual contrast If visual bright-
ness be regarded as analogous to the activity of spinal dis^
charge, and visual darkness analogous to absence of spinal
discharge, this reciprocal spinal action in the example men-
tioned has a close counterpart in the well-known experiment
where a white disc used as a prolonged stimulus leaves as visual
after-effiect a gray image surrounded by a bright ring (Hering's
** Lichthof "). The bright ring has for its spinal equivalent the
discharge from the adjacent reciprocally correlated spinal centre*
The exaltation after-effect may ensue with such intensity that
simple discontinuance of the stimulus maintaining one reflex is
immediately followed by " spontaneous '* appearance of the
antagonistic reflex. Thus the " flexion-reflex " if intense and
prolonged may, directly its own exciting stimulus is discontinued,
be succeeded by a ** spontaneous " reflex of extension, and this
even when the animal is lying on its side and the limb hori-
zontal, — a pose that does not favour the tonus of the extensor
muscles. Such a " spontaneous " reflex is the spinal counter-
part of the visual " Lichthof " To this " spinal induction," as
it may be termed, seems attributable a phenomenon commonly
met in a flexion-reflex of high intensity when maintained by very
prolonged excitation. The reflex-flexion is then frequently

FiGURH 5S. — The crowed cxtension-rcflcx. Tbia reflei was being elicited regularly by
dev^ break shocks (unipolar fanndization to ikin of opposite foi^t) at one-minute tater-
▼lU^ stimnlas and rdle^c bdng of low intensity. In the interval between B and C a
itrcniK flexioii>feAcx ot the limb r»poDding in the at»sed extension-reflex was provoked
and maintained for 4$ seooiids. The next following extension-rei)ex C shows augmenta-
tinn; thb ittfinentmtioii is also evident, though less in the next crossed reflex D. In £,
a mimite later, the augmentadon b seen to have passed oS. The signal recording the
I evoking each croased cxtcasioa-refiex is abovcL Time is marked in sacondi

brokeo at irregular intervals by sudden extetision movements
(Figs. 45, 61). It would seem» therefore, that some process
in the flexion-reflex leads to exaltation of the activity of the
arcs of the opposed extension-reflex. And electrical stimula-
tion of the proximal end of the severed nerve of the extensor
muscles of the knee (cat)» though it does not in my expe-
rience directly excite contraction of the extensors of the knee,
is on cessation often immediately followed by contraction of

them.

Pteiujts (o. " ** ll8rk*tinie *' reflex arrested by ranoiring the excitmg stimulus. Record of
mortfiicOits as before. During the period between the two marks on the sigaatl line the
idkx was tnlczTuptei! by lifting the fellaw-Umb to that yidding the tracing. On letting

1,1 i be leg bang again, the re^ex starts afresh^ but without iacreas« ticyond its previous

As examples of the rebound exaltation following on inhibi-
tion the following may also serve .*^ The so-called "mark-time "
reflex of the ** spinal " dog is an alternating stepping movement
of the hind limbs which occurs on holding the animal up so that
its limbs hang pendent It can be inhibited by stimulating the
skin of the tail. On cessation of that stimulus the stepping
movement sets in more vigorously and at quicker rate than
before (Fig. 59). The increase is chiefly in the amplitude of
the movement, but I have also seen the rhythm quickened even
by 50 per cent of the frequence.

This after-increase might be explicable in either of two ways.
It might be due to the mere repose of the reflex centre, the re-
pose so recruiting the centre as to strengthen its subsequent
action. But a similar period of repose obtained by simply

212 REFLEX SEQUENCE [Lect.

supporting one limb — which causes cessation of the reflex in
both limbs, the stimulus being stretch of the hip-flexors under
gravity — is not followed by after-increase of the reflex (Fig. 60).

Or the after-increase might result from the inhibition being
followed by a rebound to superactivity. This latter seems to
be the case. The after-increase occurs even when both hind
limbs are passively lifted from below during the whole duration
of the inhibitory stimulus applied to the tail. It is the depres-
sion of inhibition and not the mere freedom from an exciting
stimulus that induces a later superactivity. And the reflex in-
hibition of the knee-extensor by stimulation of the central end
of its own nerve is especially followed by marked rebound to
superactivity of the extensor itself

Again, the knee-jerk, after being inhibited by stimulation of
the hamstring nerve, returns, and is then more brisk than before
the inhibition (Fig. 29, p* 89).

By virtue of this spinal contrast, therefore, the extension-
reflex predisposes to and may actually induce a flexion-
reflex, and conversely the flexion-reflex predisposes to and
may actually induce an extension-reflex. This process is
qualified to play a part in linking reflexes together in a co-
ordinate sequence of successive combination.** If a reflex -arc
A during its own activity temporarily checks that of an opposed
reflex-arc B, but as a subsequent result induces in arc B a phase
of greater excitability and capacity for discharge, it predisposes
the spinal organ for a second reflex opposite in character to its
own in immediate succession to itself I -have elsewhere ^
pointed out the peculiar prominence of " alternating reflexes"
in prolonged spinal reactions. It is significant that they are
usually cut short with ease by mere passive mechanical inter-
ruption of the alternating movement in progress. It seems
that each step of the reflex movement tends to excite by spinal
induction the step next succeeding itself

Much of the reflex action of the limb that can be studied in
the " spinal " dog bears the character of adaptation to locomo-
tion. This has been shown recently with particular clearness
by the observations of Phillipson. In describing the extensor

VI] SUCCESSIVE INDUCTION 213

thrust of the limb I drew attention at the time to its signifi-
cance for locomotion. " Spinal induction " obviously tends
to connect to this extensor-thrust flexion of the limb as an
after-effect. In the stepping of the limb the flexion that raises
the foot and carries it clear of the ground prepares the antago-
nistic arcs of extension, and, so to say, sensitizes them to
respond later in their turn by the supporting and propulsive
extension of the limb necessary for progression. In such reflex
sequences an antecedent reflex would thus not only be the means
of bringing about an ensuing stimulus for the next reflex, but
would predispose the arc of the next reflex to react to the
stimulus when it arrives, or even induce the reflex without ex-
ternal stimulus. The reflex "stepping" of the "spinal" dog
does go on even without an external skin stimulus : it will con-
tinue when the dog is held in the air. The cat walks well when
anaesthetic in the soles of all four feet.

Each reflex movement must of itself generate stimuli to
afierent apparatus in many parts and organs — muscles, joints,
tendons, etc. This probably reinforces the reflex in progress.
The reflex obtainable by stimulation of the aflerent nerve of the
flexor miiscles of the knee excites those muscles to contraction
and inhibits their antagonistics : the reflex obtainable from the
aflierent nerve of the extensor muscles of the knee excites the
flexors and inhibits their antagonists.

Where a reflex by spinal induction tends to eventually bring
about the opposed reflex, the process of spinal induction is
therefore probably reinforced by the operation of any reflex
generated in the movement. This would help to explain how
it is that a reflex reaction, when once excited in a spinal animal,
ceases on cessation of the stimulus as quickly as it generally
does. Such a reaction must generate in its progress a number
of further stimuli and throw up a shower of centripetal im-
pulses from the moving muscles and joints into the spinal
cord. Squeezing of muscles and stimulation of their afierent
nerves and those of joints, etc., elicit reflexes. The primary
reflex movement might be expected, therefore, of itself to ini-
tiate further reflex movement, and that secondarily to initiate

214 REFLEX SEQUENCE [Lect.

further still, and so on. Yet on cessation of the external stimulus
to the foot in the *' flexion-reflex " the whole reflex comes usually
at once to an end. The *' scratch-reflex/' even when violent
provoked, ceases usually within two seconds of the dtscontmu-
ance of the external stimulus that provoked it

We have as yet no satisfactory explanation of this. But we
remember that such reflexes are intercurrent reactions breaking
in on a condition of neural equilibrium itself reflex. The suc-
cessive induction will tend to induce a compensatory refleXi
which brings the moving parts back again to the original posi-
tion of equilibrium.

n. Fatigue. Another condition influencing the issue of com-
petition between reflexes of different source for possession of
one and the same final common path is *' fatigue." '^ A spinal
reflex under continuous excitation or frequent repetition be-
comes weaker, and may cease altogether. This decline is pro-
gressive, and takes place earlier in some kinds of reflexes than
it does in others. In the spinal dog the scratch-reflex under
ordinary circumstances tires much more rapidly (Fig. 45) than
does the " flexion-reflex.*'

A reflex as it tires shows other changes besides decline in
amplitude of contraction. Thus, in the "flexion-reflex" the
original steadiness of the contraction decreases (Figs. 45, 61);
it becomes tremulous, and the tremor becomes progressively
more marked aud more irregular. The rhythm of the tremor
in my observations has often been about lO per second. Then
phases of greater tremor tend to alternate with phases of im-
proved contraction as indicated by some regain of original ex-
tent of flexion of limb and diminished tremor. Apart from these
partial evanescent recoveries the decline is progressive. Later,
the stimulation being maintained all the time, brief periods of
something like complete intermission of the reflex appear, and
even of a replacement of flexion by extension. These lapses
are recovered from, but tend to recur more and more. Finally,
an irregular phasic tremor of the muscles is all that remains.
It is not the flexor muscles themselves which tire out, for these,
when under fatigue of the "flexion-reflex" they contract no

- —

1 i i_ <_

r^- -^..^

t ^ t__i L

Fjouac 6i, — ne-xioTi-raflex. The reflex waj berng elidCed (by imipo!*r £u»duati<m applied
to a point in the pUntar *kin o( the outennost digit) with inteiTah of about 6o ucondi
between the end of one reactioQ and the comniencement of the nejcL A shows the com-
mcDcement of the third reactioa in the series, and this reaction was continitously elicited
for 50 Mconds ; B shows it m its latest stages The stimulation was then stopp«d for
70 seconds and Uicn reoommencjed : C shows the opening of the next reaction, the fourth
of the s«iies* In the 70 seconds interval the reflex has fully recovered from the *' fatigue **
emhibited in B. Signal above shows interruptions in primary drattt, 40 per second*
Time below in seconds. An abscissa indicates the btent periods in A and C

longer for that reflex, contract in response to the scratch-reflex
which also employ's them.

Similar results are furnished by the scratch-reflex with certain
differences in accord with the peculiar character of its dis-
charge,**^ One of these latter is the feature that the individual
beats of the scratch- re flex usually become slower and follow
each other at slower frequency (Fig. 62). Also the beats.

instead of remaining fairly regular in amplitude and frequency
tend to succeed in somewhat regular groups. The beats may
disappear altogether for a short time, and then for a short time _,
reappear, the stimulus continuing all the while (Fig. 63). Here,M
again, the phenomena are not referable to the muscle, for when i
excited through other reflex channels, or through its motor-
nerve directly ^ the muscle shows its contraction well Part of
the decline of these reflexes under electrical stimulation in the '
spinal dog may be due to reduction of the intensity of the
stimulus itself by physical polarization. That does not account
in the main for the above described effects. The graphic
record of fatigue of the flexion of the scratch-reflex obtained by
continued mechanual stimulation does not appreciably diflTer
from that yielded under electrical stimulation. The diflferent
speed of the decline due to fatigue proceeds characteristically in

PmOftl 61. — Th« scntch-rrflM toward the end of 3. loti^-majntainnl mechanic] stimutattoo
spplied to a point of the skin ojf the shoulder. The lowe&t signal line marks the time of
appliealioii of this fttimitlatioa^ which ^ when the reflex was nearly tired out, was remitted
lor about 10 seoonds aaid then repeated. The reflex then returns^ ihawing considerable
lecovery. T1«e second signal line from the bottom shows the time of application of a
iisiiliir ttimulus applied to a point of skin two catimetA^ distant from the long^maintatoed
OM^ The reflex elicited from that i)etghbo<uiiD; point ahows tittle evidence ol fatigue.
Tlae is mirked abore in ieconda.

2i8 REFLEX SEQUENCE [Lect.

different kinds of reflex, and in the same kind of reflex under
difiierent physiological conditions e. g. ** spinal shock " : this in-
dicates its determination by other factors than electrical polar-
ization. Polarization has in a number of cases been deferred as
far as possible by using equalized alternate shocks applied in
opposite directions through the same gilt needle ; this precau-
tion has not yielded results diflering appreciably from those
given by ordinary double shocks or by series of make or break
shocks of the same direction. The slowing of the beat in
" fatigue " is also against the explanation by polarization, since
merely weakening the stimulus does not lead to a slower
beat.

When the scratch-reflex elicited from a spot of sldn is
fatigued, the fatigue holds for that spot but does not implicate
the reflex as obtained from the surrounding skin.*''»*^ The
reflex is when tired out to stimuli at that spot easily obtainable
by stimulation two or more centimeters away (Fig. 63). This
is seen with either mechanical or electrical stimuli. When the
spot stimulated second is close to the one tired out, the reflex
shows some degree of fatigue, but not that degree obtaining for
the original spot This fatigue may be a local fatigue of the
nerve-endings in the spot of skin stimulated, to which in experi-
ments making use of electric stimuli some polarization may be
added. Yet its local character does not at all necessarily imply
its reference to the skin. It may be the expression of a spatial
arrangement in the central organ by which reflex-arcs arising in
adjacent receptors are partially confluent in their approach
toward the final common path, and are the more confluent the
closer together lie their points of origin in the receptive field.
The resemblance between the distribution of the incidence
of this fatigue and that of the spatial summation previously
described argues that the seat of the fatigue is intraspinal and
central more than peripheral and cutaneous ; and that it affects
the afferent part of the arc inside the spinal cord, probably at
the first synapse. Thus, its incidence at the synapse Ra — Pa
and at R/3 — PyS (Fig. 13, B, or 39, B) would explain its restric-
tions, as far as we know them, in the scratch-reflex.

The local fatigue of a spinal reflex seems to be recovered
from with remarkable speed, to judge by observations on the
reflexes of the limbs of the spinal dog. A few seconds' re-
mission of the stimulus suffices for marked though incom-
plete restoration of the reaction (Fig- 63), In a few instances
I have seen return of a reflex even during the stimulation
under which the waning and disappearance of the reflex
occurred. The exciting stimulus has usually in such cases
been of rather weak intensity. In my experience, these spinal
reflexes fade out sooner under a weak stimulus than under a
strong one. This seeming paradox indicates that under even
feeble intensities of stimulation the threshold of the reaction
gradually rises, and that it rises above the threshold value of
the weaker stimulus before it reaches that of a stronger stimulus.
This is exemplified by Fig. 64, where the scratch-reflex which

220

REFLEX SEQUENCE

has ceased to be elicited by the stimulus A is immediately
evoked — often without any sign of fatigue in its motor response
— by increasing the intensity of the stimulus (applied at the
same electrode) to A 4- a, 5 ohms having been short-circuited
from the current in the primary circuit But the occurrence of
•* fatigue '* earlier under the weaker stimulus than under the
stronger also shows that the fatigue consequent under the
weaker stimulus may often be relatively to the production of
the natural discharge greater than when a stronger stimulus is
employed. This which has been of frequent occurrence in my
observations on the leg of the spinal dog if obtaining widely in
reflex actions has evident practical importance.

It is easy to avoid in some degree the local fatigue associated
with excitation of the scratch- reflex from one single spot in the

FtGUlt:i 65. — A scratch-reflex reappearing after having lapsed completely under a stimalation
(unipolar faradization by double-induction shocks) whidi has been maintained unaltered
although the reflex It ofigtaany evoked had lapsed. The reappearances occur at irregular
intervals of long duration, *.g, ;o seconds, and the reflex on reappearance may last (or ao
seconds at a time. The stimulation was applied at ten Mparate poiQt» in the skin surface
and at each point liy tioipolar fatadization from a separate secondary circuit

skin by taking advantage of the spatial summation of stimuli
applied at different points in the receptive field. Brief-lasting
stimuli can be shifted from point to point In the field* When
this is done, a curious result has met me. The provocation of
the reflex has been made through ten separate points in the
receptive field, the distance between each member of the series

VI] FATIGUE 221

of points and the point next to it being about four centimeters.
Each point is stimulated by a double-induction shock delivered
twice a second. When this is done a series of scratch move-
ments is elicited, and continues longer than when the stimuli
are applied at the same interval, not to succeeding series of
skin points but to one point Thus three 6r four hundred
beats can be elicited in unbroken series. But the series tends
somewhat abruptly to cease. If then, in spite of the cesr
sation of the response, the stimulation be continued without
alteration during three or four minutes or more, the scratch-
ing movement breaks out again from time to time and gives
another series of beats (Fig. 65), perhaps longer than the first.
These experiments indicate that physical polarization at the
stigmatic electrode is not answerable for the fading out of the
scratch-reflex. It shows also the complexity of the central
mechanisms involved in the reflex. The phenomenon recalls
Lombard's ^^ phases of briskness and fatigue in series of records
obtained with the ergograph.

It is interesting to note certain differences between the
cessation of a reflex under " fatigue " and under inhibition.
Figs. 60 and 43 can be compared. The reflex ceasing under
inhibition is seen to fade off without obvious change in the fre-
quency of repetition of the " beats," or in the duration of the
individual beats. The reflex ceasing under fatigue is seen to
show a slower rhythm and a sluggish course for the latter beats,
especially for the terminal ones.

Among the signs of fatigue of a reflex action are several
suggesting that in it the command over the final common path
exercised for the time being by the receptors and afferent path
in action becomes less strong, less steady, and less accurately
adjusted. Under prolonged excitation their hold upon the final
common path becomes loosened. This view is supported by
the &ct that its connection with the final common path is then
more easily cut short and ruptured by other rival arcs compet-
ing with it for the final common path in question. The scratch-
reflex interrupts the flexion-reflex more readily when the latter
is tired out than when it is fresh.

222 REFLEX SEQUENCE [Lect.

In the hind limb of the spinal dog the extensor-thrust is
inelicttable during the flexion-reflex. That is to say, when the
flexion-reflex is evoked with fair or high intensity I have never
succeeded in evoking the extensor-thrust, though the flexed
posture of the limb is itself a favouring circumstance for the
production of the thrust if the flexion be a passive one. But
when the flexion-reflex is kept up by appropriate stimulation of
a single point over a prolonged time, so that it shows fatigue, the
" extensor-thrust " becomes again elicitable. Its elicitabih'ty is,
then, not regular nor facile, but it does become obtainable,
usually in quite feeble degree at first, later more powerfully. In
other words, it can dispossess the rival reflex from a common
path when that rival is fatigued, though it cannot do so when the
rival action is fresh and powerful.

Again, the crossed " extension-reflex " cannot inhibit the
flexion of the flexor-reflex under ordinary circumstances if the
intensity of the stimulation of the competing arcs be approxi-
mately equal ; but it can do so when the flexion-reflex is tired.

The waning of a reflex under long maintained excitation is
one of the many phenomena that pass in physiology under the
name of " fatigue." It may be that in this case the so-called
fatigue is really nothing but a negative induction. Its place of
incidence may lie at the synapse. It seems a process elaborated
and preserved in the selective evolution of the neural machin-
ery. One obvious use attaching to it is the prevention of the
too prolonged continuous use of a " common path " by any one
receptor.*^ It precludes one receptor from occupying for long
periods an effector organ to the exclusion of all other receptors.
It prevents long continuous possession of a common path by
any one reflex of considerable intensity. It favours the receptors
taking turn about It helps to insure serial variety of reaction.
The organism, to be successful in a million-sided environment,
must in its reactions be many-sided. Were it not for such so-
called ** fatigue," an organism might, in regard to its receptivity,
develop an eye, or an ear, or a mouth, or a hand or leg, but it
would hardly develop the marvellous congeries of all those vari-
ous sense-organs which it is actually found to possess.

VI] INTENSITY 223

The loosening of the hold upon the common path by so-
called " fatigue " occurs also in paths other than those leading
to muscle and effector organs. If instead of motor effects sen-
sual are examined, analogous phenomena are observed A
visual image is more readily inhibited by a competing image in
the same visual field when it has acted for some time than when
it is first perceived (W. Macdougall).^

One point, on a priori grounds a natural corollary from the
" principle of the common path," is indicated by the experi-
mental findings relative to the incidence of fatigue. The reflex-
arcs, each a chain of neurones, converge in their course so as to
impinge upon and conjoin in links (neurones) common to whole
varied groups — in other words, they conjoin to common paths.
This arrangement culminates in the convergence of many sepa-
rately arising arcs in the final efferent-root neurone. This neu-
rone thus forms the instrument for many different reflex arcs
and acts. It is responsive to them in various rhythm and in
various grades of intensity. In accordance with this, it seems
from experimental evidence to be relatively indefatigable?^
It thus satisfies a demand that the principle of the common path
must make regarding it

UL Intensity. In the transition from one reflex to another
a final common path changes hands and passes from one master
to another. A fresh set of afferent arcs becomes dominant on
the supercession of one reflex by the next Of all the condi-
tions determining which one of competing reflexes shall for the
time being reign over a final common path, the intensity of re-
action of the afferent arc itself relatively to that of its rivals
is probably the most powerful. An afferent arc strongly stimu-
lated is caeteris paribus more likely to capture the common path
than is one excited feebly. A stimulus can only establish its
reflex and inhibit an opposed one if it have intensity. This
explains why, in order to produce examples of spinal inhibi-
tion, recourse has so frequently been made in past times to
strong stimuli. A strong stimulus will inhibit a reflex in progress,
although a weak one will &il. Thus in Goltz's inhibition of mic-
turition in the " spinal " dog ^forcible squeeze of the tail will do

224 REFLEX SEQUENCE [Lect.

it, but not, in my experience, a weak squeeze. So likewise any
condition which raises the excitability and responsiveness of a
nervous arc will give it power to inhibit other reflexes just as it
would if it were excited by a strong stimulus. This is much as
in the heart of the Tunicate. There the prepotent spot whence
starts the systole lies from time to time at one end and from
time to time at the other. The prepotent region at one end
which usually dominates the common path is from time to time
displaced by local increase of excitability at the other under
local distension of the blood-sinuses there.

In judging of intensity of stimulus the situation of the
stimulus in the receptive field of the reflex has to be remem-
bered. One and the same physical stimulus will be weak if
applied near the edge of the field, though strong if applied to
the focus of the field.

Crossed reflexes are usually less easy to provoke, less reliable
of obtainment, and less intense than are direct reflexes. Conse-
quently we find crossed reflexes usually more easily inhibited
and replaced by direct reflexes than are these latter by those
former. Thus the crossed stepping-reflex is easily replaced by
the scratch-reflex (Fig. 52), though its stimulus be continued all
the time, and though the scratch-reflex itself is not a very
potent reflex. But the reverse can occur with suitably adjusted
intensity of stimuli.

Again, the flexion-reflex of the dog's leg is, when fully devel-
oped, accompanied by extension in the opposite leg. This
crossed extensor movement, though often very vigorous, may be
considered as an accessory and weaker part of the whole reflex
of which the prominent part is flexion of the homonymous limb.
When the flexion-reflex is elicitable poorly, as, for instance, in
spinal shock or under fatigue or weak excitation, the crossed ex-
tension does not accompany the homonymous flexion and does
not appear. But, where the flexion-reflex is well developed, if
not merely one but both feet be stimulated simultaneously with
stimuli of fairly equal intensity, steady flexion at knee, hip, and
ankle results in both limbs (Fig. (36) and extension occurs in
neither limb.*^ The contralateral part of each reflex is in-

VI] PREPOTENCY AND REFLEX SPECIES 225

2 3

FiGURB 66. ^ Diagiam (cit) of tiie predominant oncroaMd flexor-reflex of the hind limb i»>
liibitinf tiie crossed extensor-reflex otherwise obtainable by stimulation of the opposite
limb. I. The initial pose of the spinal animal ; a. The pose assnmed after stimnlation
of the left hind foot, the flexors of the left hip, knee, and ankle, and tiie extensors of the
right hip, knee, and ankle are in active contiaction ; 3. The pose assumed after stmnlta-
aeoiis stimulation of both hind feet. The extensor action of the hip, knee, and ankle
that would appear from either side as a crossed reflex is bilaterally inhibited and the an-
tagonistic flexor-reflexes bilaterally prerail.

•

hibited by the homolateral flexion of each reflex. In other
words, the more intense part of each reflex obtains possession of
the final common paths at the expense of the less intense portion
of the reflex. But if the intensity of the stimuli applied to the
right and left feet be not closely enough balanced, the crossed
extension of the reflex excited by the stronger stimulus is found
to exclude even the homonymous flexion that the weaker stimu-
lus should and would otherwise evoke from the leg to which it
is applied.

It was pointed out above that in a number of cases the
transference of control of the final common path FC fi-om one
aflerent arc to another is reversible. The direction of the trans-
ference can caeUris paribus be easily governed by making the
stimulation of this receptor or that receptor the more intense.
A factor largely determining whether a reflex succeed another
or not is therefore intensity of stimulus.**

226 REFLEX SEQUENCE [Lbct.

TV, Speoies of reflex. A fourth main determinant for the
issue of the conflict between rival reflexes seems the functional
species of the reflexes.**

Reflexes initiated from a species of receptor apparatus that
may be termed " noci-ceptive " ^^ appear to particularly domi-
nate the majority of the final common paths issuing from the
spinal cord. In the simpler sensations we experience from
various kinds of stimuli applied to our skin there can be
distinguished those of touch, of cold, of warmth, and of pain.
The adequate stimuli for the first mentioned three of these are
certainly different ; mechanical stimuli, applied above a certain
speed, which deform beyond a certain degree the resting con-
tour of the skin surface, seem to constitute adequate stimuli for
touch. Similarly the cooling or raising of the local tempera-
ture, whether by thermal conduction, radiation, etc., are ode-
quate for the cold and warmth sensations. The organs for
these three sensations have by stigmatic stimuli been traced to
separate and discrete tiny spots in the skin. In regard to skin-
pain it IS held by competent observers, notably by v. Frey ^^
and Kiesow,^^ that skin-pain likewise is referable to certain
specific nerve-endingfs. In evidence of this it is urged that
mechanical stimuli applied at certain places excite sensations
which from their very threshold upward possess unpleasantness,
and as the intensity of the stimulus is increased, culminate in
"physical pain." The sensation excited by a mechanical
stimulus applied to a touch-spot does not evoke pain, however
intensely applied, so long as the stimulation is confined to the
touch-spot. The threshold value of mechanical stimuli for
touch-spots is in general lower than it is for pain-spots; and
conversely the threshold value of electrical stimuli for touch-
spots is in general higher than it is for the spots yielding pain.
Similarly it is said that stimulation of a cold spot or of a warm
spot does not, however intense, evoke, so long as confined to
them, sensations of painful quality. But pain can be excited
not only by strong mechanical stimuli and by electrical stimuli,
but by cold and by warmth, though the threshold value of
these latter stimuli is higher for pain than for cold and warm

VI] NOCI-CEPTIVE REFLEXES 227

spots. If these observations prove correct there exist, there-
fore, numerous specific cutaneous nerve-fibres evoking pain.

A difficulty here is that sensory nerve-endingfs are usually
provided with sense-organs which lower their threshold for
stimuli of one particular kind while raising it for stimuli of all
other kinds; but these pain-ending^ in the skin seem almost
equally excited by stimuli of such different modes as mechan-
ical, thermal conductive, thermal radiant, chemical, and elec-
trical. That is, they appear aftelective receptors. But it is to be
remarked that these agents, regarded as excitants of skin-pain,
have all a certain character in common, namely this, that they
become adequate as excitants of pain when they are of such in-
tensity as threatens damage to the skin. And we may note about
these excitants that they are all able to excite nerve when applied
to naked nerve directly. Now there are certain skin surfaces
from which, according to most observers, pain is the only species
of sensation that can be evoked. This is alleged, for instance,
of the surface of the cornea — a modified piece of skin. The
histology of the cornea reveals in its epithelium nerve-endings
of but one morphological kind ; that is, the ending by naked
nerve-fibriLs that pass up among the epithelial cells. Similar
nerve-endings exist also in the epidermis generally. It may
therefore be that the nerve-endingfs subserving skin-pain are
free naked nerve-endings, and the absence of any highly evolved
specialized end-organ in connection with them may explain
their fairly equal amenability to an unusually wide range of
different kinds of stimuli. Instead of but one kind of stimulus
being their adequate excitant, they may be regarded as adapted
to a whole group of excitants, a group of excitants which has
in relation to the organism one feature common to all its com-
ponents, namely, a nocuous character.

With its liability to various kinds of mechanical and other
damage in a world beset with dangers amid which the individual
and species have to win their way in the struggle for existence
we may regard nocuous stimuli as part of a normal state of
affairs. It does not seem improbable, therefore, that there
should under selective adaptation attach to the skin a so-to-say

228 REFLEX SEQUENCE [Lect.

specific sense of its awn injuries. As psychical adjunct to the
reactions of that apparatus we find a strong displeasurable affec-
tive quality in the sensations they evoke. This may perhaps
be a means for branding upon memory, of however rudimen-
tary kind, a feeling from past events that have been perilously
critical for the existence of the individuals of the species. In
other words, if we admit that damage to such an exposed sentient
organ as the skin must in the evolutionary history of animal
life have been sufficiently frequent in relation to its importance,
then the existence of a specific set of nerves for skin-pain seems
to offer no genetic difficulty, any more than does the clotting
of blood or innate immunity to certain diseases. That these
nerve-endings constitute a distinct species is argued by their
all evoking not only the same species of sensation but the same
species of reflex movement as regards " purpose," intensity, re-
sistence to " shock," etc. And their evolution may well have
been unaccompanied by evolution of any specialized end-oi^[an,
since the naked free nerve-endings would better suit the wide
and peculiar range of stimuli, reaction to which is in this case
required. A low threshold was not required because the stimuli
were all intense, intensity constituting their harmfulness ; but re-
sponse to a wide range of stimuli of different kinds was required,
because harm might come in various forms. That responsive
range is supplied by naked nerve itself and would be cramped
by the specialization of an end-organ. Hence these nerve-
endings remained free.

It is those areas stimulation of which, as judged by analogy,
can excite pain most intensely, and it is those stimuli which,
as judged by analogy, are most fitted to excite pain which,
as a general rule, excite in the *' spinal " animal — where pain is
of course non-existent — the prepotent reflexes. If these are
reactions to specific pain-nerves, this may be expressed by
saying that the nervous arcs of pain-nerves, broadly speaking,
dominate the spinal centres in peculiar degree. Physical pain
is thus the psychical adjunct of an imperative protective reflex.
It is preferable, however, since into the merely spinal and re-
flex aspect of the reaction of these nerves no sensation of any

VI] SPECIES OF REFLEX 229

kind can be shown to enter, to avoid the term " pain-nerves."
Remembering that the feature common to all this group of
stimuli is that they threaten or actually commit damage to the
tissue to which they are applied, a convenient term for appli-
cation to them is fiocuous. In that case what from the point
of view of sense are cutaneous pain-nerves are from the point of
view of reflex reaction conveniently termed naci-^eptive nerves.

In the competition between reflexes the noci-ceptive as
a rule dominate with peculiar certainty and facility. This ex-
plains why such stimuli have been so much used to evoke
reflexes in the spinal frog, and why, judging from them, such
'* fatality " belongs to spinal reflexes.

One and the same skin surface will in the hind limb of
the spinal dog evoke one or other of two diametrically difler-
ent reflexes according as the mechanical stimulus applied be of
noxious quality or not, a harmful insult or a harmless touch.^^
A needle-prick to the planta causes invariably the drawing
up of the limb — the flexion-reflex. A harmless smooth
contact, on the other hand, causes extension — the extensor-
thrust above described. This flexion is therefore a noci-ceptive
reflex. But the scratch-reflex — which is so readily evoked
by simple light irritation of the skin of the shoulder — is
relatively mildly noci-ceptive. When the scratch-reflex and
the flexion-reflex are in competition for the final neurone com-
mon to them, the flexion-reflex more easily dispossesses the
scratch-reflex from the final neurone than does the scratch-
reflex the flexion-reflex. If both reflexes are fresh, and the
stimuli used are such as, when employed separately, evoke
their reflexes respectively with some intensity, in my experi-
ence it is the flexion-reflex that is usually prepotent (Fig.
43). Yet if, while the flexion-reflex is being moderately evoked
by an appropriate stimulus of weak intensity, a strong stimu-
lus suitable for producing the scratch-reflex is applied, the
steady flexion due to the flexion-reflex is replaced by the
rhythmic scratching movement of the scratch-reflex (Fig. 51),
and this occurs though the stimulus for the flexion-reflex
is maintained unaltered. When the stimulus producing the

230 REFLEX SEQUENCE [Lect.

scratch is discontinued the flexion-reflex reappears as before.
The flexion-reflex seems more easily to dispossess the scratch-
reflex from the final common paths than can the scratch-reflex
dispossess the flexion-reflex. Yet the relation is reversible —
by heightening the intensity of the stimulus for the scratch-
reflex or lowering that of the stimulus for the flexion-reflex.

In decerebrate rigidity, where a tonic reflex is maintaining
contraction in the extensor muscles of the knee, stimulation of
the noci-ceptive arcs of the limb easily breaks down that reflex.
The noci-ceptive reflex dominates the motor neurone previ-
ously held in activity by the postural reflex. And noci-ceptive
reflexes are relatively littie depressed by " spinal shock."

Noci-ceptive arcs are, however, not the only spinal arcs which
in the intact animal, considered from the point of view of sensa-
tion, evoke reactions rich in aflective quality. Beside those
receptors attuned to react to direct noxa^ the skin has others,
concerned likewise with functions of vital importance to the
species and colligate with sensations similarly of intense affective
quality ; for instance, those concerned with sexual functions. In
the male frog sexual clasp is a spinal reflex.^ The cord may
be divided both in front and behind the brachial region without
interrupting the reflex. Experiment shows that from the spinal
male at the breeding-season, and also at other times, this reflex
is elicited by any object that stimulates the skin of the sternal
and adjacent region. In the intact animal, on the contrary*
other objects than the female*^ are, when applied to that re-
gion, at once rejected, even though they be wrapped in the fresh
skin of the female frog and in other ways made to resemble
the female. The development of the reflex is not prevented by
removal of the testes, but removal of the seminal reservoirs is
said to depress it, and their distension, even by indifferent fluids,
to exalt it. If the skin of the sternal region and arms is re-
moved, the reflex does not occur. Severe mutilation of the
limbs and internal organs does not inhibit the reflex, neither
does stimulation of the sciatic nerve central to its section. The
reflex is however depressed or extinguished by strong chemical
and pathic stimuli to the sternal skin, at least in many cases.

VI] PREPOTENT REFLEXES 231

The tortoise exhibits a similiar sexual reflex of great spinal
potency.i^ ^^

It would seem a general rule that reflexes arising in species
of receptors which considered cu sense-organs provoke strongly
affective sensation caeteris paribus prevail over reflexes of other
species when in competition with them for the use of the ^^ final
common path.'' Such reflexes override and set aside with
peculiar facility reflexes belonging to touch organs, muscular
sense-organs, etc. As the sensations evoked by these arcs,
e.g. "pains," exclude and dominate concurrent sensations, so
do the reflexes of these arcs prevail in the competition for pos-
session of the common paths. They seem capable o( pre-eminent
intensity of action.**

Of all reflexes it is the tonic reflexes, e. g. of ordinary
posture, that are in my experience the most ecuily interrupted
by other reflexes. Even a weak stimulation of the noci-
ceptive arcs arising in the foot often suflices to lower or abolish
the knee-jerk or the reflex extensor tonus of the elbow or
knee. If various species of reflex are arranged, therefore, in
their order of potency in regard to power to interrupt one an-
other, the reflexes initiated in receptors which considered as
sense-organs excite sensations of strong aflective quality lie at
the upper end of the scale, and the reflexes that are answerable
for the postural tonus of skeletal muscles lie at the lower end
of the scale. One great function of the tonic reflexes is to
maintain habitual attitudes and postures. They form, therefore,
a nervous background of active equilibriunL It is of obvious
advantage that this equilibrium should be easily upset, so that
the animal may respond agilely to the passing events that break
upon it as intercurrent stimuli.

Therefore, intensity of stimulation, fatigue and freshness,
spinal induction, functional species of reflex, all these are phys-
iological factors influencing the result of the interaction of
reflex-arcs at a common path. It is noticeable that they all
resolve themselves ultimately into intensity of reaction. Thus,
intensity of stimulus means as a rule intensity of reaction.
Those species of reflexes which are habitually prepotent in

232 REFLEX SEQUENCE [Lect.

interaction with others are those which are habitually intense ;
those specially impotent in competition are those habitually
feeble in intensity, e. g.^ skeletal muscular tone. The tonic re-
flexes of attitude are of habitually low intensity, easily interfered
with and temporarily suppressed by intercurrent reflexes, these
latter having higher intensity. But these latter sufler fatigue
relatively early, whereas the tonic reflexes of posture can per-
sist hour after hour with little or no sign of fatigue. Fatigue,
therefore, in the long run advantageously re-dresses the balance
of an otherwise unequal conflict We can recognize in it an-
other agency working toward that plastic alternation of activi-
ties which is characteristic of animal life and increases in it with
ascent of the animal scale.

The high variability of reflex reactions from experiment to
experiment, and from observation to observation, is admittedly
one of the difficulties that has retarded knowledge of them.
Their variability, though often attributed to general conditions
of nutrition, or to local blood-supply, etc., seems far more often
due to changes produced in the central nervous organ by its
own functional conductive activity apart from fatigue. This
functional activity itself causes from moment to moment the
temporary opening of some connections and the closure of
others. The chains of neurones, the conductive lines, have
been, especially in recent years, by the methods of Golgi,
Ehrlich, Apathy, Cajal, and others, richly revealed to the
microscope. Anatomical tracing of these may be likened,
though more difficult to accomplish, to tracing the distri-
bution of blood-vessels after Harvey*s discovery had given
them meaning, but before the vasomotor mechanism was dis-
covered. The blood-vessels of an organ may be turgid at one
time, constricted almost to obliteration at another. With the
conductive network of the nervous system the temporal varia-
tions are even greater, for they extend to absolute withdrawal
of nervous influence. Under reflex inhibition a skeletal muscle
may relax to its post-mortem length,^^ i, e,, there may then be no
longer evidence of even a tonic influence on it by its motor neu-
rone. The direction of the stream of liberation of energy along

VI] PREPOTENT REFLEXES 233

the pattern of the nervous web varies from minute to minute.
The final common path is handed from some group of a plus
class of afferent arcs to some group of a minus class, or of a
rhythmic class, and then back to one of the previous groups
again, and so on. The conductive web changes its func-
tional pattern within certain limits to and fro. It changes its
pattern at the entrances to common paths.^^ The changes in
its pattern occur there in virtue of interaction between rival
reflexes, " interference." As a tap to a kaleidoscope, so a new
stimulus that strikes the receptive surface causes in the central
organ a shift of functional pattern at various synapses. The
central organ is a vast network whose lines of conduction follow
a certain scheme of pattern, but within that pattern the details
of connection are, at the entrance to each common path, muta-
ble. The gray matter may be compared with a telephone ex-
change, where, from moment to moment, though the end-points
of the system are fixed, the connections between starting points
and terminal points are changed to suit passing requirements,
as the functional points are shifted at a great railway junction.
In order to realize the exchange at work, one must add to its
purely spatial plan the temporal datum that within certain limits
the connections of the lines shift to and fro from minute to min-
ute. An example is the "reciprocal innervation" of antago-
nistic muscles — when one muscle of the antagonistic couple is
thrown into action the other is thrown out of action. This is
only a widely spread case of the general rule that antagonistic
reflexes interfere where they embouch upon the same final com-
mon paths. And that general rule is part of the general princi-
ple of the mutual interaction of reflexes that impinge upon
the same common path. Unlike reflexes Itave successive but not
simultaneous use of the common path ; like reflexes mutually re-
inforce each other on tlieir common path. Expressed teleologi-
cally, the common path^ although economically subservient for many
and various purposes^ is adapted to serve but one purpose at a
time* Hence it is a co-ordinating mechanism and prevents con-
fusion by restricting the use of the organ^ its minister^ to but one
cation at a time.

234 REFLEX SEQUENCE [Lect.

In the case of simple antagonistic muscles, and in the in-
stances of simple spinal reflexes, the shifts of conductive
pattern due to interaction at the mouths of common paths are
of but small extent The co-ordination covers, for instance, one
limb or a pair of limbs. But the same principle extended to
the reaction of the great arcs arising in the projicient receptor
organs of the head, e, g. the eye, which deal with wide tracts of
musculature as a wholes operates with more multiplex, shift of
the conductive pattern. Releasing forces acting on the brain
from moment to moment shut out from activity whole regions
of the nervous system, as they conversely call vast other regions
iiito play. The resultant singleness of action from moment to
moment is a key-stone in the construction of the individual whose
unity it is the specific office of the nervous system to perfect.
The interference of unlike reflexes and the alliance of like re-
flexes in their action upon their common paths seem to lie at the
very root of the great psychical process of " attention."

VII] REFLEXES AS ADAPTED REACTIONS 235

LECTURE VII

REFLEXES AS ADAPTED REACTIONS

Argument: Reflexes as adapted reactions. The purposes of various
type-reflexes. Shock a difficulty in deciphering the purpose of re-
flexes. Characters of spinal shock. Its incidence conflned to the
aboral side of the transection. Its diflerence in severity in difierent
reflexes and in diflerent animals. Shock referable not to the irrita-
tion of the trauma but to the cutting ofi* by the trauma of some supra-
spinal influence.

Pseudafiecdve reflexes aflbrd opportunity for determining the pain-
path in the spinal cord. This ascends both lateral columns, chiefly
the one crossed from side of stimulation. The " chloroform cry "
in decerebrate animals. Mimesis of pleasure as compared with
mimesis of pain. The bodily resonance of the emotions. The theory
of James, Lange, and Sergi. Emotional expressions in dogs deprived
of visceral and largely of bodily sensation.

It is of course as impossible to disprove as to prove that
psychical events accompany, or that they do not accompany,
the nervous reactions of the " spinal " animal. It is significant,
however, that the best-known controversy (Pfhiger, Lotze) as to
the psychical powers of the spinal cord, occurred prior to the
advent of the Darwinian theory of evolution. This latter sug-
gests how purposive neural mechanisms may arise. It furnishes
a key to the genesis and development of adapted reactions and,
among these latter, reflexes.

That a reflex action should exhibit purpose is no longer
considered evidence that a psychical process attaches to it ; let
alone that it represents any dictate of " choice " or " will." In
light of the Darwinian theory every reflex must be purposive.
We here trench upon a kind of teleology. It is widely and
wisely held that natural knowledge pursues the question " how "
rather than the question " why." The " why " involves a judg-
ment whose data lie so beyond present human experience and
comprehension that self-abnegation in regard to the desire to

236 REFLEXES AS ADAPTED REACTIONS [Lect.

attempt it is not only prudent, but to the unbiassed judgment a
necessity. Yet the question has its humbler forms as well as its
more general and ambitious.

Older writings on reflex action concerned themselves boldly
with the purpose of the reflexes they described. The language
in which they are couched shows that for them the interest of
the phenomena centred in their being regarded as manifesta-
tions of an informing spirit resident in the organism, lowly
or mutilated though that might be. Progress of knowledge has
tended more and more to unseat this anthropomorphic image
of the observer himself which he projected into the object of
his observations. The teleological speculations accompanying
such observations have become proportionately discredited.

Self-wounded in this way physiology became for a time ex-
tremely reticent about purpose, remaining simply objectively
descriptive.

The impetus given to biology by the doctrine of adaptation
under natural selection, felt so strongly by morphological studies,
seems hardly as yet to have begun its course as a motive force
in physiology. But signs begin to be numerous that such an
era is at hand.* The infinite fertility of the organism as a field
for adapted reactions has become more apparent. The purpose
of a reflex seems as legitimate and urgent an object for natural
inquiry as the purpose of the colouring of an insect or a blossom.
And the importance to physiology is, that the reflex reaction
cannot be really intelligible to the physiologist until he knows
its aim.

In general terms we may say that the effect of any reflex is
to enable the organism in some particular respect to better
dominate the environment. One often hears objection taken to
the epithets — common in writings on biology — "lower" and

* Such a "motif" seems constantly present as an undercurrent in much
recent writing in experimental pathology, notably that of Ehrlich in respect to his
suggestive "Antikorper" hypothesis. It is detectable as a principle in the fine
researches by Bayliss and Starling. Most definitely and broadly it is expressed
in the writings of A. Tschennak,^** especially in the remarkable Essay, Das
Anpassungs-problem in der Physiologie der Gegenwart, and in the contributions
of V. Uexkiill from the field of invertebrate physiology.

VII] "PURPOSE" IN REFLEXES 237

" higher " as applied to organisms, plant and animal. Such
objection seems valid if the phrase assumes that the " lower "
organism any less perfectly fulfils its " purpose " or *• design "
than does the " higher/' or in those respects in which it has
commerce with the environment is any less admirably adjusted
than is the higher. But " lower " and " higher " may be used
without any connotation of that kind. In the course of evolu-
tion a number of organisms have become so adapted to the en-
vironment as to dominate it more variously and extensively than
do other organisms. In that sense some organisms are higher
and some are lower. In that sense man is the highest organism.
And if evolution be a process of gradual and more or less un-
interrupted course it is obvious that the highest form achieved
will also be among the latest of the forms achieved. This grad-
ing of rank in the animal scale will be nowhere more apparent
than in the nervous system in its office as integrator of the in-
dividual. The more numerous and extensive the responses
made by a creature to the actions of the world around upon
its receptors, the more completely will the bundle of reflexes,
which from this standpoint the creature is, figure the complexity
of the world around, mirroring it more completely than do the
bundles of reflexes composing " lower " creatures.

The study of reflexes as adapted reactions evidently, there-
fore, includes reactions of two ranks. With the nervous system
intact the reactions of the various parts of that system, the
" simple reflexes," are ever combined into great unitary harmo-
nies, actions which in their sequence one upon another consti-
tute in their continuity what may be termed the " behaviour "
(Lloyd Morgan) of the individual as a whole. Into the intri-
cate " purposes " (adaptations) traceable in these total reactions
which constitute the creature's behaviour as a social unit in the
natural economy it is not our part to enter. Our part of the
problem is a humbler one. In the analysis of the animal's life
as a machine in action there can be split off from its total
behaviour fractional pieces which may be treated conveniently,
though artificially, apart, and among these are the reflexes we
have been attempting to decipher. We cannot but feel that

238 REFLEXES AS ADAPTED REACTIONS [Lect.

we do not obtain due profit from the study of any particular
type-reflex unless we can discuss its immediate purpose as an
adapted act

When we try to assign what we may in this restricted sense
call its "purpose" to any particular reflex, the data for an
answer are gathered in the main from one or other of the con-
ditions attaching to the reaction. The mode of the adequate
stimulus is one of these. The time-relations and spatial form
of the response are others. The broad pressure applied under
the foot-pad which seems the adequate stimulus ^^ for the " ex-
tensor-thrust " and the brief forcible straightening of the limb
which constitutes the response suggest that that reflex has its
purpose in the execution of an act in the series of movements
of stepping in the animal's locomotion. And the recent analysis
of Philippson *** demonstrates that such an act occurs in the
reflex trotting and galloping of the dog. Again, the con-
nection between the tickling irritative stimuli which seem
"adequate"** for the "scratch-reflex," and the scratching
movement itself which results, suggests that the purpose of that
reflex is a grooming of the skin to protect that organ against
parasites which infest it and would confuse its function as a
receptive surface reacting to more significant environmental
stimuli.

Grainger's ^^ conclusion was that spinal cutaneous reflexes
" are either of a preservative character or resemble the move-
ments which the functions of the organ require." From the
skin of the spinal creature reflex movements resembling those
executed by the normal individual in preening or cleansing
itself are of widespread occurrence. Together with the preen-
ing actions of the spinal fly,®* grasshopper, Astacus,^™ etc.,
there fall into this category the movement by which the spinal
frog wipes irritants from its back or head; the "nettoyage"
by the tortoise ;^^^ the posturing of the hind limbs and tail of
the spinal dog concurrently with reflex defaecation,^^ tending
to keep the body from being soiled ; and the " scratch-reflex "
and the " shake-reflex " ^^a of the spinal dog. The conjunctival
reflex protecting the cornea, essentially a cutaneous and from

VII] PROTECTIVE REFLEXES 239

the broad point of view a spinal reflex, is similarly preserva-
tive of the part whence it is initiated.

There are of course two modes of preservation, namely,
escape and defence. Parts that can move themselves seem
reflexly to employ the former. The spinal frog's foot is drawn
out of harm's way when irritated ; so also in the cat and dog.
But parts that cannot of their own motion withdraw themselves
effectively seem to invoke defensive movements from adjacent
motile parts. The spinal frog's flank, when irritated, is defended
by the hind limb, which comes up and removes the irritant from
the flank, the flank itself also shrinking away somewhat Simi-
larly in the scratch-reflex, the distant limb is brought up to the
defence of the irritated shoulder or flank. There is indeed one,
rarely exemplified, group of reflexes in which the organ is sacri-
ficed for the preservation of the rest of the individual. In certain
forms, e. g. Asterias, Cometula, Ophiurus, Arachne, Carcinus, a
limb pulled upon violently or long suddenly ruptures itself and
is shed. These actions have been shown by Fredericq to be re-
flexes, employing muscular contraction. Such reactions exhibit
well how absolutely the nervous system is adapted to minister
to the requirements of the organism as an integrated whole, and
the position of that system as a keystone in the upbuilding of
the solidarity of the individual.

But the assignment of a particular purpose to a particular
reflex is often difficult and hazardous. The difficulty is inversely
as the amplitude of the field covered by the reflex-effect. A
slight movement confined to a single limb, or a transient rise of
blood-pressure observed alone, is open to many interpretations
and admits of no security of inference. It is a fractional reaction
that may belong to any of many general reactions of varied aim.

When a reflex is elicited faintly in a spinal animal it occurs
simply at the focus, so to say, of its area of distribution, and
owing to the restricted character of its features its meaning may
be difficult or impossible to read. It is in my experience only
by repeated observations of a reflex under various circumstances
of its development that as a rule its significance becomes clear.
The accessory parts of it are often instructive concerning the

240 REFLEXES AS ADAPTED REACTIONS [Lect.

whole. In the scratch-reflex of the dog, besides th6 rhythmic
scratching movement of the hind limb, say of the right, there is
steady extension of the left hind limb, and steady extension with
some abduction of the two fore limbs. The accessory parts
of the reflex, namely those in the three limbs which arc not
scratching, are also contributory to the same eflect as in the
scratching movement in the right hind limb itself. They steady
the dog and secure the stability of its body during the perform-
ance of the scalptor act.

In the " flexion-reflex " of the hind limb excited by noxious
stimuli, e. g. a prick or a faradic current, the limb itself is drawn
up, — if weakly, chiefly by flexion at the knee ; if strongly, by flex-
ion at hip as strongly as at knee. At the same time the crossed
hind limb is thrown into action, primarily in extension, but this
is soon followed by flexion, and alternating extension and flexion
is the characteristic result The rate of this alternation is about
twice a second. That is to say, the foot which has stamped on
the thorn is drawn up out of way of further wounding, and the
fellow hind limb runs away ; and so do the fore legs when —
which is more diflicult to arrange, owing to the height of the
necessary spinal transection — they also are included, fairly free
from shock, within the " spinal " animal.

Spinal Bhock. One of the experimental difficulties in de-
ciphering the purport of a spinal reflex is the phenomenon
known as ** shock." " If in a frog the spinal marrow be divided
just behind the occiput, there are for a very short time no dias-
taltic actions in the extremities. The diastaltic actions speedily
return. This phenomenon is * shock.' " (Marshall Hall.)^

Whytt had, a century previous to Hall, drawn attention
to the same phenomenon, although assigning to it no descriptive
term. The whole of that depression or suppression of nervous
functions which ensues forthwith upon a mechanical injury of
some part of the nervous system and is of temporary nature
may be conveniently included as " shock." Goltz considered it
entirely a collection of inhibition phenomena. Among labora-
tory animals it is in the monkey that, on the whole, "spinal
shock " appears at maximum.

VII] SPINAL SHOCK 241

Spinal shock appears to take effect in the aboral direction
only.^ ** Section behind the brachial enlargement disturbs little
if at all the reactions of the fore limb, although the number
of headward running channels of conduction ruptured by such a
section is enormous. Striking instances of the absence of head-
ward spread of the depression due to " shock " are afforded by
transections abutting on the lower edge of the fifth cervical seg-
ment; these depress the respiratory activity of the phrenic
motor cells hardly at all» even momentarily. On the aboral
side of the transection depression is profound. Analogously,
the sudden cutting off of all that stream of centripetal impulses
continually pouring for conscious and subconscious elaboration
into the encephalon from the cutaneous, articular, and muscular
sense-organs of the tail, limbs, trunk, and neck, and from the
viscera, seems to disturb the reactions of the head and brain
little or not at all.

After high cervical transection, " shock " appears more
severe in the fore limbs than in the hind. For an hour or so it
may be difficult to elicit any reflex movement from skin inner-
vated behind the transection, whether by mechanical, thermal,
or electrical stimuli.

The view of Goltz and his school that " spinal shock " is
a long lasting inhibition due to irritation by trauma is not,
I think, really tenable. The argument implies, if it does
not explicitly state, that the trauma, by its damage and by its
subsequent processes of inflammatory reaction, formation of
scar-tissue, etc., acts as a stimulus, exciting inhibition that de-
presses or suppresses reflex activity in adjacent and even remote
arcs of the central nervous system. Against this explanation
militate several facts. Firstly, the shock takes effect almost
exclusively in the aboral direction. Were the mere irritative
action of the trauma the cause, it is not easy to see why the
nervous centres near the trauma should not be depressed on
either side of, for instance, a spinal transection, headward as well
as backward. Secondly, experiments of the following kind give
results difficult to reconcile with the view. When in the dog
complete transection of the spinal cord through the eighth cer-

vical segment is practised, a severe fall in the general arterial
blood- pressure ensues^ and vasomotor reflexes cannot be elic-
ited* But in the course of some days this is largely recovered
from, and after some weeks the blood*pressure will, with the
animal in the horizontal position, often be found practically
normal. When the animal is then anaesthetized and curarized,
artificial respiration being maintained* it is usually easy to obtain
on stimulation of the central ends of divided afferent or mixed
nerves, for instance of the internal saphenous nerv^e, good and
often very large vasomotor reflexes, the blood-pressure rising H
fifty millimeters and more (Fig. 67)* These reflexes upon the ^
vascular musculature are purely spinal, since the cord has been
divided just headward of the thoracic region. Then, while these
spinal vasomotor reflexes are regularly elicitable and serve as
a guide to the reflex activity of the cord behind the transection,
I have transected the cord again a couple of segments behind the
original transection. This section excites an immediate transient
rise in the arterial pressure, lasting about a minute, and succeeded
by a gradual fall The arterial pressure, then, in my experience
sinks to an equilibrium of pressure hardly lower than its mean
prior to this second transection. There is none of that deep

VII] SPINAL SHOCK 243

depression which ensued on the first trauma, though the second
trauma has been practically qua trauma a complete repetition of
the former one. If the fall of general blood-pressure be regarded
as part, and a severe part, of the ** spinal " shock which ensues
on spinal transection in the cervical region, the absence of that
fall on repeating practically the same trauma must signify that
the second trauma is not followed by the shock that followed the
first trauma. Moreover, reflex heightenings of blood-pressure
such as were regularly obtainable just prior to the second tran-
section are obtainable immediately, i. e. four minutes, after the
second spinal transection. The first trauma causes temporary
deep depression of the spinal tonus of the vascular system and
temporary abolition of vascular reflexes. The second trauma
causes practically no depression, even transient, of the re-estab*
lished tonus of the vascular system nor of the pressor spinal vas-
cular reflexes that have become similarly re-established. It may
perhaps be objected that the vascular tonus established subse-
quent to the first spinal transection is of peripheral mechanism
and outside the spinal cord itself. That that is not its main
factor is shown by the further deep depression of vascular tonus
which occurs when the spinal cord in the thoracic region is
Itself not merely transected but destroyed.

The trauma qua trauma is as severe in the first instance as
in the second. In these experiments, therefore, the " shock " is
not due to the trauma qua trauma. It seems to depend simply
on solution of continuity of nervous channels, and this solution
is practically equally great whether the actual trauma itself be
relatively slight (a clean, sharply cut transection) or relatively
severe (a contused and jagged transrupture), so long as in the
two cases it involves an equal amount of the transverse area of
the cord. The practical absence of spinal " shock " on repeti-
tion of the trauma further back is explicable by its then causing
little further aggravation of the interruption of the nervous
channels concerned with vascular tone and vascular reflexes,
those channels having already been ruptured by the previous
transection somewhat further headward.

Similarly the flexion-reflex of the hind limb, though it suffers

244 REFLEXES AS ADAPTED REACTIONS [Lect.

considerably from shock after transection of the cord in the
hinder cervical or thoracic region » when it has recovered is but
little^ and but briefly, depressed by a second transection made
behind the previous. In this case also trauma does not, there-
fore, account for the spinal shock. The shock following the
trauma is proportioned not to the mere wound, but to the num-
ber and character of the descending nerve-paths through which
the lesion breaks* Porter's *^^ * weU*known experiment on the
respiratory intraspinal path from the bulb to the phrenic n eu*
rones points to the same conclusion, ^^^M

There remains the further question as to whether sp^^^
" shock " is a phenomenon of inhibition. A reflex during ite
depression by spinal shock does not present the features it
shows when reduced by inhibition so much as features resem-

FlGURE 68 — Scratch-reflex under " spina] shock.'* SpiniJ tfansection six weeks pnvioii&ly.
The reflex wns elicited bf vigorous nK^tianlaLt stimaUtlon, electrical stimuUtioo being, i»
h usual under shock, unable to evoke it. The time of application of the sttmulns is marked
by the signal line, which also records the time in fifths of seconds. The reflex is slow to
appear, feeble and irregular, and lapses during the continuance of the stim 'elation, Th«
small waves on the base line arc due to the vigorous rubbing necessary to evoke the rcA
at ail, and are simply mechanically conveyed to the limb attached to the myograph.

VII]

SPINAL SHOCK

245

»',/^),

FtGtTUB 69. — Spinal shock. The 5cratdi-redex, as in the previous figure^ but obtained tuider
still deeper depresston of spinal shock. Time in fifths of seooods. The signal shows the
period oi mechanical exdiatioo.

bling those characteristic of it when fatigued* The scratch-
reflex under spinal shock (Figs. 68, 69) shovi^ irregularity of
rhythm, slow protracted relatively feeble beats, and speedy
onset of temporary inexcitability, features which characterize it
when nearly tired out (comp. Fig. 62, 63, Lcct, VI). So also
with the flexion-reflex of the leg. In the period of depression
by spinal shock the reflex is feeble even under strong excitation,
is relatively short-lasting, and on cessation of the exciting stim-
ulus shows little of the prolonged after-discharge that it is
prone to show at other times : it also tires out then with
abnormal rapidity. The scratch-reflex in spinal shock of
pronounced degree Kails to be el (citable by electrical stim*
uiatton at alK though still elicitable by rubbing. This indi-
cates the greater efficacy of a stimulus more nearly Uke the
adequate*

The condition of the spinal reflex-arcs in spinal shock ap-
pears to resemble a general spinal fatigue rather than an inhibi*
don. It renders diflicult and uncertain the process of conduction

246 REFLEXES AS ADAPTED REACTIONS [Lect.

along the reflex-arc as judged by the discharge from the ter-
minal neurone. This suggests a loosening of nexus between
the links of the neurone-chain composing the arc ; a defect of
transmission at the synapse. Such a conception of the disorder
accords well with the suggestion of v. Monakow**^ that a " dia-
schists " takes place between the conducting cells, his *' schalt-
zellen " failing to perform their normal function as connecting
elements.

I think, therefore, that spinal shock is neither due to irritation
by trauma, nor in the main a phenomenon of inhibition. The
rupture of certain aborally conducting paths appears to induce
it Which these paths exactly are is matter for research. After
cervical transection separating the cord from the bulbar vaso-
motor centre, the phenomenon might be attributable to the
invariably severe fall of general arterial pressure. But this
cannot be the chief explanation, for: (i)the head does not par-
ticipate in the '' shock," although participating in the low blood-
pressure; (2) with post-thoracic transection the body region
distal to the spinal lesion exhibits shock as severe as after cervical
transection, though there is no fall of blood-pressure ; (3) tran-
section anterior to the bulbar vasomotor centre but posterior to
the pons leaves the blood-pressure unreduced but the spinal
shock severe.

The shock is more profound in the monkey than in other ani-
mals observed in the laboratory. This might suggest a cerebral
origin for the paths implicated. But ablation of the hemispheres
does not induce anything like the depth of spinal depression in-
duced by transections behind the pons. The much severer
character of the depression when the transection passes behind
the pons indicates an aborally directed influence from some
nucleus of the pontine or midbrain system, driven probably
by the great cranial receptors of otic labyrinth and eye, rein-
forced by impulses from the cord itself. The great influence
on spinal centres of a cranial mechanism in this region, driven
by the otic labyrinth, is illustrated by Ewald's ^^ proposed name
*' tonus labyrinth " for the end-organ of the octavus nerve. The
great severity of shock in the monkey would accord with high

VII] SPINAL SHOCK 247

exercise of function of this apparatus in an animal endowed with
such variety and range of skeletal movement

In the monkey and in man spinal shock is not only pecul-
iarly intense but peculiarly long lasting. The withdrawal from
the isolated cord of influences it is wont to receive from
centres further headward may induce an alteration of trophic
character in spinal cells — an *' isolation dystrophy " ^ — visible,
it may be, as Nissl's chromatolysis. This " isolation-dystrophy "
ensuing on shock would add itself as a longer lasting, in some
elements perhaps a permanent, depression. Certainly spinal
transection is followed in the monkey by longer lasting " shock "
— included in which I suspect is " isolation-dystrophy " — than
in other animal types observed in the laboratory. My results
in monkeys bore out that which Bastian,^^ Bowlby,^^ and
Bruns,**^ contrary to previous observers, have described as the
typical condition in man after spinal injury completely severing
the cord. Thus, I found the knee-jerk sometimes inelicitable
during a month or so after midthoracic transection in the monkey,
whereas in the rabbit its abeyance lasts usually but ten minutes
or a quarter of an hour.

It is noteworthy that spinal shock takes efiect on just
those tissues which waste when the synaptic nervous system is
destroyed — namely the skeletal muscles. Where the primitive
diffuse nervous system, the nerve-net, exists, as in the visceral and
vascular musculature, neither" spinal shock" nor atrophy occur
consequently to spinal transection. In the skeletal muscles the
'* spindles " do not waste.^*^ Jamin found that the disuse-
wasting of the muscles in spinal dogs which had been daily
exercised in reflex actions in my laboratory was much less than
in other dogs he examined. The organs on which "shock'*
falls least heavily are those which suffer least even after exsec-
tion of the spinal cord itself

The deeper depression of reaction into which the higher ani-
mal as contrasted with the lower sinks when made " spinal,"
appears to me ^^ significant of this, that in the higher types,
more than in the lower, the great cerebral senses actuate the
motor organs and impel the motions of the individual

248 REFLEXES AS ADAPTED REACTIONS [Lect.

"Spinal shock" does not fall upon all reflexes with equal
severity. Noci-ceptive reflexes suffer relatively slightly. In
the dog, afler spinal transection in the posterior part of the
cervical region, the reflexes acting on the muscles of the hind
limb show less severe and shorter-lasting depression in regard
to the flexion-reflex and in regard to the scratch-reflex than
to the extensor-thrust. That may explain why a number of
observers have not obtained any homonymous reflex of ex-
tension in the spinal mammal. The crossed extension-reflex,
which is really a part of the great reflex of which homony-
mous flexion is the more prominent feature, recovers from
spinal shock earlier than does the extensor-thrust

There is variability in the order of recovery of the various
spinal reflexes of the dog from spinal shock. Occasionally
the scratch-reflex returns as early as the flexion-reflex. Al-
though usually in the hind limbs of the spinal dog no extensor
rigidity develops, in some individuals it does so. The limbs
are kept extended at knee and ankle even to a deg^ree that it
is difficult to break through by the inhibition accompanying
elicitation of the flexion-reflex on stimulation of the foot
It is not difficult to see how this may come about. Some
incidental circumstance determining the preponderance of some
passive attitude of the limbs during the early days succeeding
the lesion may, by its influence on the interaction of the re-
covering spinal arcs, impress an unwonted reflex habit upon
the limbs. It is not uncommon to find, especially in the spinal
monkey, I think, differences in the reflex condition of the right
and left limbs, even although the spinal transection has given
a perfectly symmetrical spinal lesion. Such inequality or dis-
similarity of the spinal reflexes right and lefl does not neces-
sarily afford any evidence that the spinal lesion is asymmetrical.
Intercurrent circumstances suffice to impress slightly different
reflex habits on the two limbs, and in one and the same in-
dividual the reflex habits of each limb may vary somewhat from
period to period {cf, Lewandowsky on the production of
hemiplegic contracture).

Local Bign in reflezoB. The locus of the stimulus plays an

VII] LOCAL SIGN IN REFLEXES 249

important part in determining the nature of the reflex evoked.
This influence of the location of the stimulus on the resulting
reflex movement has been one of the features most studied
in reflex action. It furnishes a large part of the direct evidence
of the " purposive " character of spinal reflexes.^

The rule of spatial proximity g^ven above partly expresses
the influence of this factor. Much that was mentioned regarding
long irradiation illustrates it further. Though the importance
of the locus is high when broadly taken, it does not appear obvi-
ous as attaching to small differences of location in a more or
less homogeneous receptive fleld or area. Yet in such a field
the reflexes, though similar, are demonstrably not identical.^
In the spinal monkey, excitation of the outer edge of the planta
while causing dorso-flexion at ankle, in doing so generally
brings the peronei into play more than is the case when the
flexion is excited from the inner edge of the planta ; then tibialis
anticus predominates, causing some inversion. In the frog,
excitation of the skin of the dorsal aspect of the knee, and of
the ventral aspect respectively, alike evoke flexion at hip, knee,
and ankle, but in the former case the foot is somewhat everted,
in the latter somewhat inverted. We must allow that the cen-
tripetal impulses, although they yield no sensation, yet possess, to
borrow a term from the psychologist, "local sign." In the naked
eye Medusa, called on account of its localizing reflexes Tiaropsis
indicans^ the manubrium deflects itself towards the stimulated
part of the nectocalyx; its tip is brought with precision to
meet the concurrently contracted inbent portion of the necto-
calyx. If one point of the nectocalyx be irritated, and while the
manubrium is applied to that point, then another, the manubrium
will leave the first point and move over to the second. In this
way it may be made to indicate successively a number of points
of irritation. " After a series of such irritations the manubrium
subsequently continues for some time to visit first one and then

1 The "rule of spatial proximity" offers an ezplanatioD for many of those
minor differences obtaining in broadly similar reflex movements, there being a
tendenqr for the muscles belonging to the immediate spinal vicinity of the ridn
stimulated to respond in preponderant degree ; similarly in the scratch-reflez.

2SO REFLEXES AS ADAPTED REACTIONS [Lect.

another of the points which have been irritated." ^ A cut be-
tween the base of the manubrium and the point of irritation in
the bell destroys the localization, though movement occurs
toward some part of the quadrant of the bell containing the site
of stimulus; but the accuracy of the localization is reduced.
The reaction recalls the bending of the tentacles of Drosera®
in the direction needful to reach the seat of stimulation on the
leaf. The headless bee stings in response to stimulation of the
under-surface pretty accurately at the site of irritation.^^ In
the "spinal" crayfish, if one leg is caught it is flexed and drawn
up, and soon all the others, if the leg is not released, are
brought round it and push at the hand holding the limb.^^ The
yellow clover-fly will, after decapitation, stand cleaning its wings
with its hind legs, and clean its ''three pairs of legs, rubbing
them together in a determined manner, and raising its fore legs
vainly in air as if searching for its head to brush up."^ But
in Astacus the accuracy of localization is much impaired on
the crossed side by cutting the cross commissures combining
the ganglia most closely concerned with the reaction.** This
recalls the effect of the tangential cut in the nectocalyx of
TiaropsisP

In a reflex reaction exhibiting " local sign " in the above sense,
the afferent impulses involved are divisible into several groups
according to their place of origin. There must be (i) a group
originated at the seat of stimulus, (2) a group initiated in the
motor and mobile organs reflexly set in action, and (3) in
some cases a group arising at the distant spot to which the
movement is directed. Regarding this last group an experi-
ment illustrates its extinction without extinction of the *' local
sign." Thus, in Astacus''^ after section of the nerve-cords
behind the mouth, when, therefore, the hind creature without
mouth has lost all nervous connection with the front creature
possessing the mouth, food given to the claws of the hind
creature is still at once and accurately carried by them to the
mouth, and this latter may refuse to take the morsel brought.
In the grasshopper,^"^ after extirpation of the supra and
suboesophageal ganglia (entire brain), the front leg is pro-

VII j PSEUD AFFECTIVE REFLEXES 251

tracted, and in the normal way catches the antenna, and the
usual movements of cleaning the antenna go on, although the
antenna has entirely lost its innervation owing to the destruc-
tion of the brain. Regarding the second mentioned group of
afferent impulses, H. E. Hering^ has made the interesting
observation that the " cleansing" reflex of the spinal frog which
brings the foot to a seat of irritation on the dorsal or perineal
skin is accurately executed after severance of the afferent spinal
roots of the limb itself. In the same way the bulbo-spinal frog
brings the fore limb to the snout when the snout is stimulated
after section of the afferent roots of the fore limb. The scratch-
reflex I find executed without obvious impairment of direction
or rhythm when all the afferent roots of the scratching hind limb
have been cut through. In the execution of these spinal reflexes,
therefore, the most important afferent factor as regards " local
sign " is the afferent channel from the place of initiation of the
reflex.

PseadaffectiTe reflexes. If we turn to reflex-effects excited
by nocuous stimulation of the skin but having for their field
of development a wider conjunction of reflex-arcs and con-
sequently a wider mechanism of reflex expression, the reflex
response seems to indicate yet more clearly the " purpose " of
the reflex.

If from the cat under deep chloroform narcosis the cerebral
hemispheres and part of the thalami be removed, on relaxing
the narcosis a number of motor reactions can be observed
against the background of '^decerebrate rigidity** ^^^ Among
these reactions are some mimetic movements simulating ex-
pression of certain affective states. These ** pseudaffective "
reflexes Woodworth and myself^* have endeavoured to use for
elucidation of the spinal path conducting those impulses that,
were the brain intact, would, we may presume, evoke " pain."
The search for such a path is, as regards channels from skin, a
search for a path as specific as those of the special senses.
The truncation of the brain of the mammal at the mesenceph-
alon annihilates the neural mechanism to which the affective
psychosis is adjunct. But it leaves fairly intact the reflex motor

252 REFLEXES AS ADAPTED REACTIONS pJBCT.

machinery whose concurrent action is habitually taken as out-
ward expression of an inward feeling. When the expression
occurs it may be assumed that, had the brain been present, the
feeling would have occurred. Pain is the psychical adjunct of
a protective reflex. A spinal translesion which prevents occur-
rence of the expression in response to a stimulus that previously
excited the expression has therefore been regarded by us in the
following experiments to be sucli as would, were the brain
present, induce analgesia in regard to that stimulus. Even
apart from that assumption, it is clear that such a lesion can be
used for determining the conducting path of a noci-ceptive
reaction. The spinal path concerned with the forward trans-
mission of these impulses can therefore be designated not
merely a headward path, but, having regard to the character of
the reaction, the headward path for noci-ceptive (p. 266) reactions.
The reflex-effect observed has presented the following de-
ments : diagonal cyclic movements of the limbs as in preces-
sion (sometimes producing progression), turning of head and
neck toward the point stimulated; opening of the mouth,
retraction of the lips and tongue, movement of the vibrissae ;
snapping of the jaw ; lowering of the head ; opening of the eye-
lids, dilatation of the pupils ; vocalization angry in tone (snarl-
ing), sometimes plaintive; and with these a transient increase
of arterial blood-pressure. These reactions appear not only in
combination, but sometimes singly or in small combinations.
The most readily elicitable are movements of the vibrissae,
opening of the mouth with retraction of the tongue, and lower-
ing of the head ; but though in some cases vigorous and prompt
they never amount to an effective action of attack or escape.
A characteristic feature of their ineffectiveness is their brief
duration. The movement, even when most vigorous and prompt,
dies away rapidly, to be succeeded in some cases by a few
weaker repetitions, each in succession weaker and more tran-
sient than the last. Thus, the movements of the head may
recur three or four times in response to a single stimulus,
or the vocalization be repeated in a diminishing series for a
minute or so.

VII] SPINAL PATH FOR PAIN 253

Our method has been to compare by mean3 of the above
reaction the effect of two stimuli symmetrically but succes-
sively applied on opposite sides of the body, after a semisection
or other lesion of the spinal cord headward of the entrance of
the nervepath stimulated.

After semisection at the 13th thoracic level the pseud-
affective reaction was obtained by stimulation of either sciatic
trunk, but more vigorously and promptly from the nerve of the
side of the semisection : from this nerve also the reaction was
evoked by weaker faradization. This indicates that the head-
ward pathway taken by the impulses eliciting the vocal and
other pseudaffective reactions is from the hind limb both crossed
and uncrossed, but is more largely crossed. From our experi-
ments we are able to exclude the dorsal spinal column as the
main path of conduction. Sections of both dorsal columns made
no appreciable difference in the reaction to the stimulus ; neither
did faradization of them evoke the reaction. The median por-
tion of the ventral column has sometimes been trespassed on
in making the semisection of the opposite side ; this extension
of the lesion has not prevented the reaction from occurring.
In one case the whole gray matter of both halves of the cord
was found at the autopsy to be heavily infiltrated and ploughed
up with extravasated blood at the level of the semisection and
for several millimetres both ahead and behind it It must have
been largely, if not completely, thrown out of function. Yet the
pseudaffective reaction remained very brisk.

If, therefore, neither the dorsal nor the ventral column nor
the gray matter affords the pathway for the noci-ceptive (algesia)
impulses, the lateral column alone is left to them. This con-
clusion is confirmed by direct experiment After transection
of one lateral column alone the pseudaffective reaction is elicited
from either lateral half of the body behind the lesion; after
further section of the opposite lateral column, all pseudaffective
reaction at once ceases to be elicitable from either half of the
body behind the lesion. It is probable that in the posterior
thoracic and lumbar segments this headward path is that already
signalized by A. Frohlich and myself^*' as inhibiting, under

254 REFLEXES AS ADAPTED REACTIONS [Lect.

direct faradization, the rigidity of the triceps brachii in the
decerebrate cat.

We concluded from our observations (i) that the lateral
column furnishes the headward path in the spinal cord for
noci-ceptive (algesic) arcs; (2) that each lateral column con-
veys such impulses from both lateral halves of the body, and
somewhat preponderantly those from the crossed half; and (3)
that this is true for these arcs whether they be traced from
skin^ muscles^ or viscera.

It is noteworthy that the *' chloroform " or " ether cry," that
peculiar vocalization emitted by men and animals during certain
stages of anaesthetization, was often uttered *'• by decerebrate
cats during the continued administration of the anaesthetic after
decerebration. This vocalization does not necessarily mean an
imperfect anaesthetization or any persistence of consciousness,
since in our animals the whole cerebrum and the "'tween "-brain
had been ablated when the administration of the vapour still
evoked the vocalization typically.

The crying of the young infant has been noticed in hemi-
cephalic children to be strong and of usual character even
in total absence of the cerebrum and midbrain (Sternberg and
Latzko^). These malformed infants seem to react as do
normal of the same age to stimuli that, judging from adult
experience, are unpleasant. They cry or whimper, pucker the
mouth, and retract the head. The drawing down of the angles
of the mouth and the drawing down of the lower lip seem in-
dicative of pain: pouting of the lips — a mimetic movement
common also in the young gorilla, chimpanzee, and macaque —
seems to indicate displeasure. Nothnagel and others incline to
regard the optic thalamus as the seat of the nerve-centres of mi-
metic expression. Experiments on animals and the observations
on hemicephalic children just referred to seem to contradict
this. But we must remember that various grades of mimetic
movements exist — and some seem phylogenetically much older
than others. The congenital have to be distinguished from
those that are acquired. The mimesis of the infant is not that
of the adult. The latter may depend on the thalamic region;

VII] BODILY RESONANCE OF EMOTIONS 255

much of the former seems to be a reaction for which neither the
forebrain nor midbrain are necessary. In the decerebrate cat
we could never evoke such mimesis as might, had the cerebrum
been present, have been indication of pleasurable sensation.
Never, for instance, could purring be elicited, although its oppo-
site, snarling, was obtained so easily. The decerebrate dogs
observed by Goltz ^^ responded to almost all forms of skin stim-
ulus by growling, as if in resentment. Thus, they did so when
lifted from their cage to be fed each midday. No mimesis
indicative of pleasure was ever obtained from them. Pain centres
seem to lie lower than pleasure centres. As far as I can find
from reference to books and the experience of coUeag^ues,
*' pain " is unknown as an aura in cortical epilepsy, or at least is
of equivocal occurrence. No region of the cortex cerebri has
been assigned to pain. Such negative evidence g^ives perhaps
extraneous interest to the ancient view, represented in modem
times by Schopenhauer, that pleasure is an absence of pain.

Bodily resonanoe of amotions. Some sensations are neutral
or devoid of affective tone, while others are rich in affective
tone. The development of these latter is closely connected
with the orig^in of the coarser emotions. A physiological inter-
est attaches to these states of emotion since certain reactions
of the bodily organs are, as is well known, characteristic of
them. That marked reactions of the nervous arcs regelating
the thoracic and abdominal organs and the sldn contribute
characteristically to the phenomena of emotion has been com-
mon knowledge from time immemorial.

To this bodily resonance of the emotions has in recent years
been assigned by some authorities a prominent rdle in the mech-
anism of the production of the emotional state itself in certain
of the coarser emotions. Instead of the emotional state begin-
ning, as Ladd*^ puts it, as " a sort of nerve storm in the brain,
whence there descends an excitement which causes commotion
in the viscera and vascular regions — thus secondarily inducing
an organic reverberation " — the view has been advanced that the
cerebral and psychological processes of emotion are secondary
to an immediate reflex reaction of vascular and visceral orgrans

256 REFLEXES AS ADAPTED REACTIONS [Lect.

of the body suddenly excited by certain stimuli of peculiar
quality.

Of points where physiology and psychology touch, the place
of one lies at ** emotion." Built upon sense-feeling much as
cognition is built upon sense-perception, emotion may be re-
garded almost as a** feeling," — a feeling excited, not by a simple
little-elaborated sensation, but by a group or train of ideas. To
such compound ideas it holds relation much as does '' feeling"
to certain species of simple sense-perceptions. It has a special
physiological interest in that certain visceral reactions are
peculiarly colligate with it Heart, blood-vessek, respiratory
muscles, and secretory glands take special and characteristic part
in the various emotions. These viscera, though otherwise re-
mote from the general play of psychical process, are affected
vividly by the emotional. Hence many a picturesque metaphor
of proverb and phrase and name — " the heart is better than the
head," anger " swelb within the breast," " Richard Coeur de
Lion." It was Descartes ^ who first promoted the emotions to
the brain. Even last century Bichat wrote,' " The brain is the
seat of cognition, and is never affected by the emotions, whose
sole seat lies in the viscera." But the brain is now thought to
be a factor necessary in all higher animals to every mechanism
whose working has consciousness as an adjunct.

What is the meaning of the intimate linkage of visceral
actions to psychical states emotional? To the ordinary day's
consciousness in the healthy individual the life of the viscera
contributes little at all, except under emotion. The perceptions
of the normal consciousness are rather those of outlook upon
the circumambient universe than inlook into the microcosm of the
" material me." Yet heightened beating of the heart, blanching or
flushing of the blood-vessels, the pallor of fear, the blush of shame,
the Rabelaisian effect of fright upon the bowel, the secretion by the
lacrymal gland in grief, all these are prominent characters in the
pantomime of natural emotion. Visceral disturbance is evidently
a part of the corporeal expression of emotion. The explanation
is a particular case in the problem of movements of expression
in general. The hypothesis of evolution afforded a new van-

VII] EMOTIONAL EXPRESSION 257

tage point for study of that question. The bodily expressions
of the " coarser or animal emotions " are largely common to
man and higher animals. This point of view is exemplified
by Darwin's argument ^ concerning the contraction of the
muscles round the eyes during screaming. " Children, when
wanting food or when suffering in any way, cry out loudly,
as do the young of most animals, partly as a call to their
parents for aid, and partly from any great exertion serving
as relief. Prolonged screaming inevitably leads to the engorg-
ing of the blood-vessels of the eye; and this will have led
at first consciously and at last habitually to the contraction
of the muscles round the eyes in order to protect them." ^ Her-
bert Spencer wrote :^ ** Fear, when strong, expresses itself in
cries, in efforts to hide or escape, in palpitations and tremblings ;
and these are just the manifestations which would accompany an
actual experience of the evil feared. The destructive passions
are shown in a general tension of the muscular system, in gnash-
ing of the teeth and protrusion of the claws, in dilated eyes and
nostrils, in growls : and these are weaker forms of the actions
that accompany the killing of prey." In short, the bodily ex-
pressions of emotion are instinctive actions reminiscent of ances-
tral ways of life.

They must have an explanation the same in kind as that of
other instinctive movement. There is no real break between
man and brute even in the matter of mental endowment The
instinctive bodily expressions of emotion arose, in the opinion
of those quoted above, as attitudes and movements useful to
the animal for defence, escape, seizure, embrace, etc. These as
survivals have become symbolic for states of mind. Hence
an intelligible nexus between the muscular attitude, the pose
of feature, etc., and the emotional state of mind. But between
action of the viscera and the psychical state the nexus is less
obvious. This latter connection adds a difficult corollary to
the general problem.

The fact of the connection is on all hands admitted, but as to
the manner of it opinion is at issue. Does (i) the psychical part
of the emotion arise and its correlate nervous action then excite

2S8 REFLEXES AS ADAPTED REACTIONS [Lect.

the viscera? Or (2) does the same stimulus which excites the
mind excite concurrently and per se the nervous centres ruling
the viscera? Or (3) does the stimulus which is the exciting
cause of the emotion act first on the nervous centres ruling the
viscera, and their reaction then generate visceral sensations;
and do these latter, laden with affective quality as we know they
will be, induce the emotion of the mind? On the first of the
three hypotheses the visceral reaction will be secondary to
the psychical, on the second the two will be collateral and
concurrent, on the third the psychical process will be secondary
to the visceral.

To examine the last supposition first. It is a view which in
recent years has won notable adherents. Professor James
writes : m* • " Our natural way of thinking about these coarser
emotions (j-g- "grief, fear, rage, love") is that the mental per-
ception of some fact excites the mental affection called the
emotion, and that this latter state of mind gives rise to the bodily
expression. My theory, on the contrary, is that the bodily
changes follow directly the perception of the exciting fact, and
that our feeling of the same changes as they occur IS the emotion''
** Every one of the bodily changes, whatsoever it be, is FELT
acutely or obscurely, the moment it occurs. If the reader has
never paid attention to this matter, he will be both interested
and astonished to learn how many different local bodily feelings
he can detect in himself as characteristic of his various emo-
tional moods." '* If we fancy some strong emotion and then try
to abstract from our consciousness of it all the feelings of its
bodily symptoms we find we have nothing left behind, no * mind-
stuff* out of which the emotion can be constituted, and that a
cold and neutral state of intellectual perception is all that
remains." " If I were to become corporeally anaesthetic, I
should be excluded from the life of the affections, harsh and
tender alike, and drag an existence of merely cognitive or intel-
lectual form."

Professor Lange ^^ traces the whole psycho-physiology of
emotion to certain excitations of the vasomotor centre. For him,
* The italics and emphasizing capitals are quoted as in the original.

VII] PHYSIOLOGICAL REACTIONS 259

as for Professor James, the emotion is the outcome and not the
cause or the concomitant of the organic reaction ; but for him the
foundation and corner-stone of the organic reaction is as to physi-
ological quality vascular, namely, vasomotor. Emotion is an out-
come of vasomotor reaction to stimuli of a particular kind. This
stimulus induces a vasomotor action in viscera, skin, and brain.
The change thus induced in the circulatory condition of these
organs induces changes in the actions of the organs themselves,
and these latter evoke sensations which constitute the essen-
tial part of emotion. It is by excitation of the vasomotor
centre, therefore, that the exciting cause, whatever it chance
to be, of emotion produces the organic phenomena which as
felt constitute for Lange the whole essence of emotion. The
teaching of Professor Serg^ *®» ^^ closely approaches to that of
Lange.

The views of James, Lange, and Sergi have common to them
this, that the psychical process of emotion is secondary to a
discharge of nervous impulses into the vascular and visceral
organs of the body suddenly excited by certain peculiar stimuli,
and that it depends upon the reaction of those organs. Pro-
fessor James's position in the matter is, however, not wholly like
that of Professor Lange. In the first place, he does not consider
vasomotor reaction to be primary to all the other organic and
visceral disturbances that carry in their train the psychological
appanage of emotion ; and Professor Sergi, though more nearly
in harmony with Lange, agrees with James so far. In the sec-
ond place, Professor James seems to distinctly include other
" motor " sensations and centripetal impulses from musculature
other than visceral and vascular, among those which casually
contribute to emotion. Thirdly, he urges his theory as one
completely competent only for the " coarser " emotions, among
which he instances " fear, anger, love, and grief." For Lange
and Sergi the basis of apparition of all feeling and emotion is
physiological, visceral, and organic, and has its seat for the
former authority exclusively, and for the latter eminently, in the
vasomotor system.

To obtain some test of this view is not difficult by experi-

26o REFLEXES AS ADAPTED REACTIONS [Lect.

ment.^^ Appropriate spinal and vagal transection removes
completely and immediately the sensation of the viscera and of
all the skin and muscles behind the shoulder (Fig. 70). The
procedure at the same time cuts from connection with the organs
of consciousness the whole of the circulatory apparatus of the
body. I have had under observation dogs in which this has
been carried out. I will cite an animal selected because of

Figure 70. — Diagram to indicate the extent of the parts still retaining sensHiTity after
combined spinal and yagosjrmpathetic nerve sections described in the teat. The extent
of sldn surface left sentient is delimited by the continuoas (not dotted) lines in the
figure. The limit of ''deep," i.e, muscular, articular, etc, sensitirity also corresponds
with this line. But the limit to which the respiratory and alimentary tracts still re-
tained sensation is shown by dotted outlines of the larynx and upper part of oesophagus.
From anatomical data it is presumed that the trachea and oesophagus had been de-
prived of all sensitivity somewhere about those levels. The curved line behind the chest
indicates the diaphragm as the only muscle behind the shoulder still retaining afferent
nerves.

markedly emotional temperament Affectionate toward the
laboratory attendants, one of whom had her in charge, toward
some persons and toward several inmates of the animal house
she frequently showed violent anger. Her ebullitions of rage
were sudden. Their expression accorded with a description
furnished by Darwin.^ Besides the utterance of the growl,
*' the ears are pressed closely backwards, and the upper lip is
retracted out of the way of the teeth, especially of the canines."
The mouth was slightly opened and lifted, the eyelids widely
parted, the pupils dilated. The hair along the mid-dorsum, from

VII] ANGER 261

close behind the head to a point more than half-way down the
trunk, became rough and bristling.

The reduction of the field of sensation in this animal by the
procedure above mentioned produced no obvious diminution of
her emotional character. Her anger, her joy, her disgust, and
when provocation arose, her fear, remained as evident as even
Her joy at the approach or notice of the attendant, her rage at
the intrusion of a cat with which she was unfriendly, remained
as active and thorough. But among the signs expressive of rage
the bristling of the coat along the back no longer occurred. On
the other hand, the eyes were well opened and the pupils dis-
tinctly dilated in the parox3^m of anger. Since by the transec-
tion the brain had been shut out from discharg^ing impulses via
the cervical sympathetic the dilatation of pupil may have occurred
by inhibition of the action of the oculomotor centre.

The coming of a visitor whose advent months before had
elicited violent anger, again provoked an exhibition of wrath
significant as ever. The expression was that of aggressive rage.
The animal followed each movement of the stranger as though
of an opponent, growling viciously. A cat with which she was
never friendly, and a monkey new to the laboratory, approach-
ing too near the kennel, excited similar outbursts. No doubt
was left in our minds that sudden attacks of violent anger were
still easily excited. But she also gave evidence daily that she
had the accession of joyous pleasure and delight she had always
shown at the approach of the attendant the first thing of a morn-
ing, or at feeding time, or when caressed by him, or encouraged
by his voice.

Few dogs, even when very hungry, can be prevailed upon to
touch dog's flesh as food. Almost all turn from it with signs of
repugnance and dislike. I had strictly refrained from testing
this animal previously with regard to disgust at dog's flesh
offered in her food. Flesh was given her daily in a bowl of
milk, and this she took with relish. The meat was cut into
pieces rather larger than the lumps of sugar usual for the break-
fast table. It was generally horse-flesh, sometimes ox-flesh.
We proceeded to the observation thus: the bowl was placed

262 REFLEXES AS ADAPTED REACTIONS [Lect.

by the attendant in the corner of the stall with milk and meat in
every way as usual ; but the meat was flesh from a dog killed on
the previous day. Our animal eagerly drew itself toward the
food ; it had seen the other dogs fed, and evidently was itself
hungry. Its muzzle had almost dipped into the milk before it
suddenly seemed to find something there amiss. It hesitated,
moved its muzzle about above the milk, made a venture to take a
piece of the meat, but before actually seizing it stopped short and
withdrew again from it. Finally, after some further examination
of the contents of the bowl (it usually commenced by taking out
and eating the pieces of meat), without touching them, the
creature turned away from the bowl and withdrew itself to
the opposite side of the cage. Some minutes later, under
encouragement from us to try the food again, it returned to the
bowl. The same hesitant display of conflicting desire and dis-
gust was once more gone through. The bowl was then removed
by the attendant, emptied, washed, and horse-flesh similarly
prepared and placed in a fresh quantity of milk was oflered in
it to the animal. The animal once more drew itsMf toward the
bowl, and this time began to eat the meat, soon emptying the
dish. To press the flesh upon our animal was of no real avail
on any occasion ; the coaxing only succeeded in getting her to,
as it were, re-examine but not to touch the morsels. The
impression made on all of us by the dog's behaviour was that
something in the dog's flesh was repulsive to her, and excited dis-
gust unconquerable by ordinary hunger. Some odour attaching
to the flesh seemed the source of its recognition.

It would be instructive for judging the part played by the
cerebral hemisphere in the reactions of coarser emotion did we
know whether repugnance to dog's flesh as food would be ex-
hibited by a dog after ablation of the cerebral hemispheres.
Even the primitive emotions seem to involve perception — seem
little other than sense-perceptions richly sufl'used with aff*ec-
tive tone. Goltz's^ dogs after ablation of the hemispheres
evinced signs of hunger, namely restlessness when their feed-
ing hour was deferred. When a little quinine (bitter) was
added to the sop of meat and milk the morsels taken into

VII] DISGUST 263

the mouth were at once rejected. No inducement or scold-
ing modified this unfailing and unhesitating rejection. Goltz
adds that he threw to his own house dog a piece of the
same doctored meat The creature wagged its tail and took it
eagerly, then pulled a wry face, and hesitated, astonished. But
on a look of encouragement from its master the dog swallowed
it. He overcame his instinctive rejection of it, and thus, as Goltz
remarks, by his self-control gave proof of the intact cerebrum
he possessed.

Fear appeared clearly elicitable (as also in dogs with spinal
cervical transection only. Fig. 71). The attendant approaching
from another room of which the door stood open, chid the dog
in high scolding tones. The creature's head sank, her gaze
turned away from her advancing master, and her face seemed to
betray dejection and anxiety. The respiration became unquiet,
but the pube never changed its rate.

In the dog, after transection of the spinal cord, the regions
of the body which have been thus made purely " spinal " con-
tinue their litjp in many respects normally. The hairy " coat "
changes in spring. The oestral periods recur even when the
transection is performed in puppyhood, and altogether headward
of the spinal origin of the sympathetic system, e,^. at the cervi-
cal segment Goltz ^ observed successful impregnation and
parturition, and suckling completed without obvious abnormal-
ity. In my own observations **•* the natural instinct of the
female toward the male at oestrum was seen indubitably dis-
played after the spinal cord had been transected in the cervical
region more than a year previously.

It may be objected to these experiments that although the
animals expressed emotion they may yet have fe/t none. Had
their expression been unaccompanied by, and had they not led on
to, trains of acts logically consonant with their expressed emotion,
that objection would have weight. Where the /acus of anger is
followed by actions of advance and attack with all appearance of
set purpose, I find it difficult to think that the perception initiat-
ing the wrathful expression should bring in sequel angry conduct
and yet have been impotent to produce " angry feeling."

REFLEXES AS ADAPTED REACTIONS [Lec

Figure 71. -^ Record of the arterial pressure in a docj forty-one days after iptnjtt transection
at the 7th cerrical s^ment. The arterial pressure is high and good m spite ol the tran-
section, the p«rlod of vatomotor shock ha>ring pas»ed by. For the short period marked
bf the ugoa] the fioiMof the vibrator of an mductorium sounded and was heard by the
aoiiiial. The point of the signal marked nearly S mm. further to the right than did the
kyinogrmph poi. The inhibition of the heart is shown by the oscillations 00 the kymo-
graph trace. The line marked " Z^ro of B. P/' aignihea the height of the tero of tt
manometer recording the arterial preuure.

VII] BODILY REINFORCEMENT OF EMOTION 265

A weaker point in such experimentation is that although the
visceral and vascular and much of the muscular mechanism of
emotional expression was cut off, a small but notable fraction
of the latter, namely the facial, still remained open to react on
the centres with which consciousness is colligate.

Nevertheless, in view of these observations the vasomotor
theor)*^ of the production of emotion becomes, I think, untenable,
also that visceral sensations or presentations are necessary to
emotion. A mere remnant of all the non-projecting or affective
senses was left, and yet emotion persisted. If I understand it
aright, Professor James' and Lange's theory lays stress on organic
and visceral presentations, but re-presentations of the same
species might no doubt be put forward in their place. That
would be a different matter. To exclude the latter hypothesis,
the deprivation of vascular and organic sensation might have to
date from a very early period of the individual life. Professor
Lloyd Morgan writes *^ in respect to the above experiments,
" The avenues of connection were closed after the motor and
visceral effects had played their part in the genesis of the emo-
tion on the hypothesis that the emotion is thus generated. Al-
though new presentative data of this type were thus excluded,
their re-presentative afler-effects in the situation were not ex-
cluded." But it is noteworthy that one of the dogs under
observation had been deprived of its sensation when only nine
weeks old. Disgust for dog's flesh could hardly arise from the
experience of nine weeks of puppyhood in the kennel.

We are forced back toward the likelihood that the visceral
expression of emotion is secondary to the cerebral action occur-
ring with the psychical state. There is a strong bond between
emotion and muscular action. Emotion " moves " us, hence the
word itself. If developed in intensity, it impels toward vigor-
ous movement. Every vigorous movement of the body, though
its more obvious instrument be the skeletal musculature of the
limbs and trunk, involves also the less noticeable co-operation
of the viscera, especially of the circulatory and respiratory.
The extra demand made upon the muscles that move the frame
involves a heightened action of the nutrient organs which sup-

26S REFLEXES AS ADAPTED REACTIONS [Lect.

ply to the muscles the material for their energy. This increased
action of the viscera is colligate with this activity of muscles.
We should expect visceral action to occur along with the muscu-
lar expression of emotion. The close tie between visceral action
and states of emotion need not therefore surprise us.

That emotion is primarily a cerebral reaction obtains sup-
port from observations where the hemispheres of the brain have
been removed. Goltz observed a dog kept many months in
that condition. It on no occasion gave any evidence of joy or
pleasure in commerce either with man or beast. Of sexual
emotion it never gave a sign. Anger or displeasure, Goltz
says, it repeatedly expressed, both by gesture and by voice.
Save for these expressions of displeasure, it was indifferent and
supremely neutral to its surroundings. We are, of course, in
observations such as this, hopelessly cut off from introspective
help. It can be urged that the expression of emotion might
be provocable and nevertheless the psychical emotion remain
absent. On such an hypothesis the same stimulus which excited
the mind must excite concurrently and per se motor centres
producing movement appropriate to an affective process in the
mind. This is not improbable. All sensations referred to the
body itself rather than interpreted as qualities of objects in the
external worlds tend to be tinged with " feeling." Sense-organs
which initiate sensations tinged with feeling tend to excite motor
centres directly and imperatively. Hence in animals reduced to
merely spinal condition stimuli calculated to produce pain
(although, of course, unable to do so in a spinal animal) evoke
movements appropriate for escape from or removal of the stimu-
lus applied. Now " feeling" is implicit in the emotional state;
the state is an " afifective state." In the evolution of emotion
the revival of " feelings " pleasureable and painful must have
played a large part Hence the close relation of emotion with
sense-organs that can initiate bodily pain or pleasure, and hence
its connection with impulsive or instinctive movement There
is no wide interval between the reflex movement of the spinal
dog whose foot attempts to scratch away an irritant applied to
its back — both leg and back absolutely detached from con-

VII] EMOTIONAL REACTION 267

sciousness — and the reaction of the decerebrate dog that turns
and growls and bites at the fingers holding his hind foot too
roughly. In the former case the motor reaction occurs, although
the mind is not even aware of the stimulus, far less percipient
of it as an irritant. The action occurs, and plays the panto-
mime of feeling; but no feeling comes to pass. In the latter
case the motor reaction occurs and is expressive of emotion ;
but it is probably the reaction of an organic machine which
can be started working, though the mutilation precludes the
psychosis.

And with the gesture and the attitude will occur the visceral
concomitant It would be consonant with what we know of
reflex action, if the spur that started the muscular expression
should simultaneously and of itself initiate also the visceral
adjunct reaction. It is almost impossible to believe that with
the mere stump of brain that remained to Goltz*s dog there
could be any elaboration of a percept All trace of memory
seemed lacking to the creature. Yet though not evincing
other emotion, anger it showed as far as expression can yield
such revelation. Fear, joy, affection seem, therefore, in the ex-
perience of this skilled observer of animal behaviour, to demand
higher nervous organization than does anger. Be that as it may,
the retention of its expression by Goltz's dog indicates that by
" retrogradation " the complex movement of expression has in
certain emotions passed into a simpler reflex-act Under the
canalizing force of habit the determining motives become, even
in impulsive acts, weaker and more transient. The external
stimulus originally aroused a strongly affective group of ideas,
which operated as a motive, but now it causes a discharge of the
act before it can be apprehended as an idea. The impulsive
movement of a " lower," " coarser," so-called " animal " emotion,
has in this case become an automatic reflex no longer neces-
sarily combined with the psychical state whence it arose, of
which it is normally at once the adjunct and the symbol.

In view of these general considerations and of the above
experiments, we may with James accept visceral and organic
sensations and the memories and associations of them as con-

268 REFLEXES AS ADAPTED REACTIONS [Lect.

tributory to primitive emotions, but we must regard them as
reinforcing rather than initiating the psychosis. Organic and
vascular reaction, though not the actual excitant of emotion,
strengthen it. This is the kernel of the old contention about
actuality of emotion in the art of the artist. Hamlet's descrip-
tion of the actor as really moved by his expression may be
accepted as an answer.

Conversely, as Lloyd Morgan *^^ writes, "Whatever be the
exact psychological nature of the emotions, it may be regarded
as certain that they introduce into the conscious situation ele-
ments which contribute not a little to the energy of behaviour."
A feeling of pain and a protective reflex movement — either
of defence or escape — are concurrent in the reaction of an
animal to a hurtful stimulus of the skin. Reflexes to which
emotion is adjunct are not only prepotent (Lect. VI) but are
imperative, that is, volition cannot easily suppress them. Now,
the morphological disposition of the nervous channels is such
that the physiologist can, by suitable severance of the spinal
path to the brain, sunder the reflex movement from the sensa-
tion, leaving the former effect but perforce annulling the latter.
The former is, however, in absence of the latter, not left un-
altered; it is abnormally reduced, especially in duration (7/.
supra, p. 252). The pseudaffective reactions indicative of re-
sentment and defence are, after ablation of the cerebral cortex,
short-lived, the simulacra of mere flashes of mimetic passion. No
cerebral reverberation descends to prolong and develop further
the protective movement set going as a spinal reflex. This con-
trasts strongly with the fairly normal course that the headward
part of the reflex, after loss of its vascular and visceral fields,
runs. The difference argues that the reverberation from the
trunk, limbs, and viscera counts for relatively little, even in the
primitive emotions of the dog, as compared with the cerebral
reverberation to which is adjunct the psychical component of
emotional reaction.

VIII] FUNCTIONAL TOPOGRAPHY 269

LECTURE VIII

SOME ASPECTS OF THE REACTIONS OF THE MOTOR

CORTEX

Argument: Remarkable that electrical stimuli applied to the organ of
mentality yield with regularity certain localized movements from
certain restricted areas of its sm-fiace. Functional topography of
"motor" cortex in the chimpanzee, orang-utan, and gorUla. The
cerebral fissures, not functional boundaries. The anthropoid ape has
a direct pyramidal tract like that of man. Recovery of function not
due to symmetrical part of opposite hemisphere taking on supple-
mental work. Inhibition as elicitable from the cortex. Reciprocal
innervation of antagonistic eye-muscles. Reciprocal inhibition in
other muscular groups. Seat of the inhibition subcortical in these
cases. Reciprocal innervation in willed movements. Preponderant
representation in the " motor " cortex of the same movements as are
preponderantly elicitable as local reflexes from the cord and bulb.
Scanty representation of certain movements as cortical and local
spinal reactions alike. Appearance under strychnine and tetanus
toxin of these movements reversing the normal direction of the
preponderance. This due to these agents transmuting reciprocal
inhibition into excitation. Decerebrate rigidity. A system of tonic
innervation in action. Strychnine and tetanus toxin augment this
innervation. Hughlings Jackson's "co-operative antagonism" of
paired systems of innervation, one tonic, the other phasic De-
cerebrate rigidity and hemiplegic rigidity. The relation of the
cortex to receptor organs ; the pre-emiment representation in it of
the ** distance-receptors."

We shall now venture a glance at certain reactions of the
cerebral hemisphere itself; our survey must be circumscript
for several reasons. By use of such methods as we are em-
ploying, artificial excitation and so on, and under such ob-
servations as these allow, namely the initiation under narcosis
of muscular movements or the recording of their immediate
defects from normal movement, little light is given in regard
to much that goes on in an organ whose chief function
is mentality itself. Our expectation must be modest, for

270 REACTIONS OF THE MOTOR CORTEX [Lect.

modest assuredly must be the achievement reached by such
means in a problem of such a nature. The very poverty of the
achievement is itself an indication that the methods pursued by
the physiologist successfully in other spheres of his study are
here confronted with problems to which they are far less
suited. It is not that I esteem lightly the labours of the many
distinguished workers in this field. As far as the methods
referred to can avail, it is to the skill with which they have
been used that we owe what knowledge we have of the topo-
graphical representation of movement in the various fields of
cerebral cortex. We have only to remember how much more
numerous the physiological facts concerning the cerebral cortex
are to-day than prior to the experiments of Hitzigand Fritsch*^
and of Ferrier,*^ following on the observations of Broca,®
Hughlings Jackson,^ and Bastian.*^ Experiment had failed to
get evidence of localization of function in the cortex of the
hemispheres, though in microscopic structure that great sheet
of gray matter presents such similarity to nervous formations
regarded as nerve-centres elsewhere. Progress of knowledge in
regard to the nervous system has been indissolubly linked with
determination of localization of function in it This has been
so from the time of the Bell ^^-Magendie " discovery of the dif-
ference of function in the two spinal roots, and Flourens' ^® de-
limitation of the respiratory centre in the bulb. The discovery
of localization of function in parts of the cortex has given the
knowledge which now supplies to the student charts of the
functional topography of the brain much as maps of continents
are supplied in a geographical atlas. The student looking over
the political map of a continent may little realize the complexity
of the populations and states so simply represented. We
looking at the brain chart of the text-book may never forget
the unspeakable complexity of the reactions thus rudely sym-
bolized and spatially indicated.

If we may be allowed an ^ /rr'^n consideration it is this, —
that although it is not surprising that such territorial subdivision
of function should exist in the cerebral cortex, it is surprising
that by our relatively imperfect artifices for stimulation we

VIII] ANTHROPOID MOTOR CORTEX 271

should be able to obtain clear evidence thereof. The neurone
chains that together build up the nervous system are in the
architecture of that system so arranged that the longest of them
all tend to pass through the cerebral cortex. Every increase
in the number of links composing a nerve-cell chain seems to
increase greatly the uncertainty of its reactions under artificial
excitation. With increase in number of links goes increase in
numbers of side branches and connections. The difficulty of
getting long chains of nerve-cells to react in a regular way under
artificial stimulation seems greatly enhanced by the multiplica-
tion of the side connections. The momentary condition of any
cell-chain is in part a function of the condition at the moment
of all the other cell-chains with which it is connected. The
cortex cerebri might therefore well have been expected to yield
under artificial stimulation only extraordinarily inconstant results.
To Hitzig and Fritsch, and to Ferrier, we owe the pregnant
demonstration that as regards the motor region this expectation
is not well founded.

It is only of the reactions of the Rolandic area of the cortex
that I shall venture to speak. Ferrier showed that the applica-
tion of faradic currents to that cortex excites with great regu-
larity movements which vary in distribution as the electrodes
are moved from place to place, but remain within limits constant
under repeated application of the stimulus to any one and the
same spot Ferrier's mode of indicating the topographical
arrangement of the reactions he obtained is seen in his well-
known diagrams of the cortex. His motor centres, as he
termed them, were marked in his figures by circular areas.
"The areas have no exact line of demarcation from each
other, and where they adjoin stimulation is apt to produce
conjointly the effect peculiar to each."^ He showed these
motor centres to extend forward over the frontal lobe, produc-
ing there movements of the eyeballs. Regarding their exten-
sion round and over the upper edge of the hemisphere and
down upon the mesial surface he noted them in the marginal
convolution. "This convolution in the parieto-frontal region
gave rise to movements of the head and limbs apparently

272 REACTIONS OF THE MOTOR CORTEX [Lect.

similar to those already obtained by stimulation of the corre-
sponding regions on the external surface." '^

This original research by Ferrier ranks among the classics
of experimental neurology and physiology. It has been followed
by a number of kindred contributions from workers whose names
are familiar to us all, — Albertoni, Schafer, Munk, Luciani, Tam-
burini, Paneth, Beevor, Horsley, Mott, Ballance, Mann, and others.
The detailed knowledge of the localization has been largely
based on the cerebral cortex of the common ape, the macaque.
It was an interesting step further when Beevor and Horsley ^
published observations on the localization of the motor functions
in the central cortex of an orang-utan. Their experiment
long remained the single one for which an anthropoid species
had been laid under contribution. It exercised a notable influ-
ence on the scheme of motor localization adopted as probably
obtaining in the brain of man.

Of the three or four species of anthropoid apes that are
known, most authorities agree that it is the gorilla which pos-
sesses the most highly developed cerebrum ; next to it probably
stands the chimpanzee, and a little below the chimpanzee comes
Simia satyrus^ the orang-utan. But there are gre^t individual
differences, and the simpler examples of chimpanzee brains seem
inferior in development to well-developed examples of the brain
of the orang.

A. S. Griinbaum^^^ and myself have obtained observa-
tions on cerebral localization in the several species of anthropoid
apes. In the chimpanzee the scheme of topography we find
existent is illustrated by the accompanying Figures 72 and 73.

The so-called motor area occupies unbrokenly the whole
length of the precentral convolution, and in most places the
greater part or the whole of its width. It extends into the depth
of the central sulcus, occupying the anterior wall, and in some
places the floor, and in some extends even into the deeper
part of the posterior wall of the fissure. We have examined
more than forty hemispheres, but have never found the motor
area extend indubitably to the free face of the post-central
convolution. This delimitation agrees remarkably with the

VIII] ANTHROPOID MOTOR CORTEX 273

original results obtained by Hitzig^ on the brain of the monkey,
Innuus rhesus.

At the upper mesial edge of the hemisphere the motor area
extends round and down upon the mesial face of the hemisphere,
but we have not found it reach the calloso-marginal fissure.
The anterior limit of the motor region is in great part not coin-
cident with any fissure. The front portion of the region usually
dips into and across the upper part of the superior precentral
fissure, and lower down it not infrequently dips into the inferior
precentral fissure. Occasionally the front edge of the region
dips into almost the whole length of the superior precentral
sulcus. It is not the extent of the motor area which appears
to be variable, the variant is the sulcus itself. The great variety
of individual pattern exhibited by the convolutions and sulci
in these richly convoluted brains gives opportunity for studying
critically the claim of value of these fissures as landmarks In
the topography of the cortex. From this point of view their
use for strict localization is small. Not only are the extremes
of pattern exhibited by the convolutions extraordinarily dif-
ferent one from another, but the frequency of the individual
variation is so great that hardly a pair can be found in which
the existent convolutions are, when compared with the mi-
nuteness applicable to functional centres, really closely alike.
Schafer, in his important contribution to the physiology of
the motor cortex in 1887,"^ pointed out that the fissures of the
cortex do not mark in any sense the boundaries of the functional
areas of the organ. Our examination of the anthropoid brains
we have worked through convinces us that not only do the
fissures of the frontal region not mark physiological boundaries^
but that they are not closely reliable even as landmarks for
the functional topography. Their relation is too inconstant v.
Monakow^^ has found the same uncertainty in the calcarine
fissure in respect to visual cortex. The degree to which these
fissures are subject to individual variation and the frequency of
their asymmetry in the two hemispheres stands in contrast with
the constancy from individual to individual and greater bilateral
symmetry which holds good for the arrangement of the func-

274 REACTIONS OF THE MOTOR CORTEX [Lect.

QpenkMf

Sulca&centt^U^.

H^s^fcmon

Figure 72 (from Griinbauin and Sherrington). — Bnun of a chimpantee {TrogUdytts
niger). Left hemisphere viewed from side and abore so as to obtain as far as possible the
configuration of the sulcus centralis area. The figxire involves, nevertheless, considerable
foreshortening about the top and bottom of sulcus centralis. The extent of the ^ motor "
area on the free surface of the hemisphere is indicated by the black stippling, which
extends back to \ht sulcus centralis. Much of the "motor" area is hidden m suld;
for instance, the area extends into the sulc. centralis and the sulc. frecentraUs^
also into occasional sulci which cross the precentral gyrus. The names printed large on
the stippled area indicate the main regions of the '* motor " area; the names printed small
outside the brain, indicate broadly by their pointing lines the relative topography of
some of the chief subdivisions of the main regions of the "motor" cortex. But there
exists much overlapping of the areas and of their subdivisions which the diagram does
not attempt to indicate.

The shaded regions, marked " EYES," indicate in the frontal and occipital regions
respectively the portions of cortex which, under faradization, yield conjugate movements
of the eyeballs. But it is questionable whether these reactions sufficiently resemble those
of the ** motor " area to be included with them. They are therefore marked in vertical
shading instead of stippling, as is the "motor" area. S. F. = superior frontal sulcus.
S. Pr. = superior precentral sulcus. I. Pr. = inferior precentral sulcus.

tional centres. A practical outcome of this is that it is essential
for accurately detailed localization, when the opening through the
skull is of moderate size, not to trust to the anatomical details of

VIII] TOPOGRAPHY IN THE CHIMPANZEE 275

Adc.c&lcarin,

tttdd-

FiGURB 73 (from Granbauira and Sherrington). — Bndn of a diimpaasee {TrogMyUs
^*g^)* Left hemisphere; mesial surface. The extent of the "motor** area on the
free surface of the hemisphere is indicated by the black stippling. On the stippled area,
" LEG *' indicates that morements of the lower limb are directly represented in all the
regions of the ** motor '* area visible from this aspect. Such mutual overlapping of the
minuter sulxiiTisions exists in this area that the diagram does not attempt to exhibit
them. The pointing line from " Anus, etc,** indicates broadly the position of the area
whence perineal movements are primarily elidtable.

SmU. central. — central fissure. Sulc, caUarin, = calcarine fissure. Sulc. parul0
•ccip, = parieto-oodpital fissure. Sulc, caUof marg» = caUoso-marginal fissure. SmU,
precentr. marg, = pre-cential fissure.

The single italic letters mark spots whence, occasionally and irregularly, movements of
the foot and leg (//), of the shoulder and chest (/), and of the thumb and fingers {h)
have been evoked by strong faradization. Similarly the shaded area marked ** EYES "
indicates a field of free surface of cortex which under faradization yields conjugate move-
ments of the eyeballs. The conditions of obtainment of these reactions separates them
from those characterizing the ** motor *' area.

the exposed cerebral surface, but to obtain orientation in the topo-
graphy by apph'cation of the electrodes and observation of the
movement, if any, which is excited. In our early experiments
we thought to obtain much help by having at hand a brain
of the same species already experimented upon and thought to
save time in recording the results of the fresh experiment upon
chart outlines prepared from the specimen already worked

276 REACTIONS OF THE MOTOR CORTEX [Lbct.

opon. But the variatioii of the convolutioas from individual
to individual has been too great to allow of these expedients.
As an exception to the above general rule two landmarks of
relative constancy are the genua of the sulcus centralis^ the
Rolandic fissure. In the chimpanzee and gorilla the genua
are two (Fig. 72) ; the upper, opposite the junction between
leg area and arm area, may be termed the cruro-brachial ; the
lower, between arm area and face area, may be termed the
brachuhfaciaL In the orang there is in addition a third genu,
which from its relation to the functional topography may be
called the labia-linguaL In the orang the facial area of the
cortex is considerably longer from above down than in the
chimpanzee or gorilla.

It is a general belief that for excitation of the cortex in man
there is needed an intensity of faradism much greater than that
sufficing for the cortex of the monkey. Actually comparing the
excitability of the cortex of the anthropoid with that of the
bonnet monkey by employing exactly the same current in each
case, we found the excitability as measured by the least intensity
of current required to evoke motor reaction practically the
same in the anthropoid and in the lower ape.*^ The motor
cortex of the anthropoid, though undoubtedly far more complex
in many ways than that of the lower ape, remains as readily
amenable to electric stimulation. Gushing of Baltimore and
Krause of Berlin find this holds good also for the human
brain, and that it is not necessary to employ strong faradization.
In the majority of the anthropoids upon which we have experi-
mented cortical epilepsy has been quite easily provoked, just as
it is in the small monkeys.

In the precentral gyrus, the sequence of representation of
the musculature starting from below upward follows broadly
that known for the lower apes. The sequence runs — tongue,
jaw, mouth, nose, ear, eyelids, neck, hand, wrist, elbow, shoulder,
chest, abdomen, hip, knee, ankle, toes, and perineal muscles.
It is noticeable that movements of the eyeballs do not occur
in this list As did Beevor and Horsley in their orang,^ so
we in the chimpanzee, gorilla, and orang find a frontal area

VIII] FACILITY OF EXCITATION 277

extending into the middle and inferior frontal convolutions
excitation of which gives conjugate deviation of the eyeballs
to the opposite side. We find this area separated from the
area }nelding the other movements by an intervening space.
We find this intervening space broken, however, by small areas
whence movements of the eyeballs can be elicited, partially
bridging between it and the upper facial region on the
precentral convolution.

The sequence of representation of movement which we note
follows a plan more in accordance with the order of the spinal
series of segments than that hitherto obtained. Between the
place of representation of shoulder and that of hip is an area
which, next to the shoulder, yields unilateral movement of the
chest muscles, and, next to the hip, yields unilateral movement
of the abdominal muscles, and furthest upward lies a focus for
perineal muscles.

In our experience, in accord with the original observations
by Hitzig^ on the lower apes, the electrodes when placed upon
the surface of the post-central convolution fail to evoke any
obvious effect, though when placed with an even weaker current
upon the precentral they evoke the regular reaction. In our
experience, though small lesions in the precentral convolution
caused marked paralyses and descending spinal degenerations,
similar and larger lesions in the post-central did not produce
even temporary paralysis nor any unequivocal degeneration.

With regard to the degenerations, it is noteworthy that from
a hand-area lesion the spinal pyramidal degeneration shows in
some chimpanzees a ventral direct pyramidal tract of size not
obviously inferior to that of a man. But this ventral direct
tract does not appear to be present in all individual chim-
panzees, — a fact agreeing with Flechsig's^ discovery of its
variability in man. The hand-area lesion gives a heavy degen-
eration of the homolateral pyramidal tract in the lateral colunm
on the same side as the cerebral lesion.

As regards symptoms resulting from the cortical lesions,
extirpation of a great part if not of the whole of the hand area
from the right hemisphere caused an immediate severe crossed

278 REACTIONS OF THE MOTOR CORTEX [Lect.

brachioplegria without the slightest sign of paresis in either face
or leg. The paresis affected the fingers most ; these were kept
helplessly semi-extended, the wrist being dropped. The elbow
seemed little if at all affected, but the shoulder seemed dis-
tinctly paretic, there being difficulty in raising or abducting the
upper arm. The paresis diminished quite rapidly, and in six
weeks' time the animal had in large measure recovered the
usefulness of the limb.

A lesion in the leg-area similarly caused temporary paresis
of the opposite leg, especially in the toes and at the ankle-joint
The lesion was smaller and the recovery more rapid than with
the arm-area lesion. The knee-jerk, which showed no alteration
under the arm-area lesion, here showed exaltation immediately,
that is, a quarter of an hour after the leg-area lesion. Weeks
later, when the paresis to inspection had passed off, the knee-
jerk still exhibited greater briskness on the crossed side.

We have seen often confirmed what our predecessors ^^ with
the orang have well pointed out, namely, the greater integration
of localized representation of movements in the anthropoid as
compared with the lower ape. There is not one of the fingers
that we have not seen move separately and alone under excita-
tion of certain points of the cortex ; again, isolated movement of
the pinna of the ear, of the tip of the tongue, rare in the lower
monkeys, are easily obtainable in the anthropoid.

As to the extent of the so-called motor area, from our obser-
vations we think it probable that in the anthropoid brain as
much of that area lies hidden from the surface in the sulci as
is actually exposed on the free surface on the convolutions.
Nevertheless we indorse the opinion expressed by Beevor and
Horsley that the so-called motor area in the anthropoid brain
forms a smaller fraction of the total surface than it does in the
lower types of monkey. If it has grown in extent — as undoubt-
edly it seems to have done — other regions belonging to those
so-called "silent" fields whence electric stimulation excites no
obvious response have increased still more. It is especially
with the exploration of that great inexcitable field that research
has to deal. The discoveries of Flechsig, v. Monakow, D^jerine,

VIII] EXTENT OF MOTOR CORTEX 279

Mott, Campbell, the Vogts, and others yearly advance further
in the problem.

The results on the gorilla ^'^ confirm those we have obtained
on the chimpanzee, though in the gorilla and orang in my
experience eyeball movements have been elicited from a larger
field of the frontal cortex than in the chimpanzee, an upper
area yielding eye movement on a level with the hand area being
more readily discoverable.

That the free surface of the post-central convolution belongs
to the motor cortex we have not found. Brodman*** and
Campbell*^ have since called attention to marked structural
differences between the cortex respectively behind and in front
of the central sulcus. The arrangement of the fibres and the
character of the cells is different. Ramon-y-Cajal,^* using the
Golgi method, and Flechsig^'^ following the myelinization, had
also previously drawn distinction between the structure of the
two convolutions divided by this great fissure ; and observations
by Mott, A. Tschermak, and others had indicated an especially
close connection of the post-central gyrus with ascending pre-
sumably afferent paths. Evidence of this last by excitation
methods is of course difficult to obtain, but nothing in our
experiments is contrary to it, and some occasional results that
have come before me in experimenting by excitation lend them-
selves to such an explanation. To enter upon these here would
lead too far from our main interest now.

It is natural to inquire whether reciprocal innervation is
exemplified by reactions from the cortex. That inhibition of
muscular contraction is obtainable by artificial excitation of the
cortex was early noted by Bubnoff and Heidenhain ^ in the dog
and by Exner ^ in the rabbit. I have myself worked chiefly
with the monkey.^"* •^ In that animal the ocular axes are par-
allel, and the setting of the eyeball in the orbit is such that
the tensions of its connections, apart from unequal activity of
its extrinsic muscles, are in equilibrium when the globes are
approximately parallel. That is their primary position. This
can be shown in various ways. Thus, if the III, IV, and VI
nerves are all severed the eyeballs assume this primary position.

28o REACTIONS OF THE MOTOR CORTEX [Lect.

If one eyeball be then rotated by the finger or fixation forceps
to right or left or up and down a considerable resistance is felt,
and on letting it go the globus springs back at once to the
primary position. So also in chloroformization. In the early
stage of chloroformization the eyes enter various positions of
squint — in the monkey a very usual position is bilateral diver-
gence upward and outward. When the narcosis has become
profound, the eyes revert to approximate parallelism in the
primary position. On being then displaced by the finger they
at once swing into the primary median position again. So also
immediately after death, before rigor mortis has set in.

If III and IV cranial nerves of one side, e. g. left, have been
severed, so that rectus extemus remains the only unparalyzed
ocular muscle, appropriate excitation of the cortex cerebri pro-
duces conjugate movement of both eyes towards the opposite
side, I. e, from left toward right, the left eye travelling however
only so far as the median line. Inhibition of the tonus and of
the active contraction of recttis extemus can thus be elicited
from the cortex. The reaction is obtainable from all that por-
tion of the cortex which on excitation gives conjugate lateral
deviation of the eyes, i, e. from the area discovered by Ferrier"*
in the frontal region, and from that discovered by Schafer ^^* in
the occipital region.

This inhibition is obtainable from the frontal area after com-
plete removal of the occipital lobe. It is conversely obtainable
from the occipital area after complete removal of the frontal area.
After a deep frontal section across the hemisphere and into the
lateral ventricle (partly entering the internal capsule) so as to
sever occipital from frontal cortex in the manner practised by
Munk and Obregia,^ the reaction is obtainable undiminished
from both the frontal and from the occipital areas separately.

The cortex is not essential to the reaction. It is obtain-
able from the corona radiata underiying the frontal cortex after
complete ablation of the frontal cortex itself. It is obtainable
from the corona radiata running downwards and forwards from
the occipital cortex after free removal of the latter. It is obtain-
able by direct excitation of the internal capsule itself. From

VIII] RECIPROCAL INNERVATION FROM 281

the internal capsule it is elicitable at two distinct places, one in
front of the other behind the genu of the capsule. It is obtain-
able by excitation of the cross-section of corpus callosum about
3-5 millimeters behind the genu; also from corpus callosum
at the splenium. The section was laid bare as in Mott and
Schafer's^"* second method. The reaction as obtained from
corpus callosum has in my hands proved comparatively irregu-
lar. It is evident that the action of arrest may take place, in
centres which are subcortical.

E. H. Hering and myself^^ made observations on limb
movements elicited from the cortex and found evidence of a
similar co-ordination in regard to them. As in the experi-
ments of Bubnoff and Heidenhain,*" the degree of narcotiza-
tion formed an important condition for the observations. The
narcosis must not be too profound. It is best, starting with
the animal in a condition of deep etherization, to allow that
condition gradually to diminish. As this is done it almost
constantly happens that at a certain stage of anaesthesia the
limbs, instead of hanging slack and flaccid, assume and maintain
a position of flexion at certain joints, notably at elbow and hip.
This condition of tonic contraction having been assumed, the
narcosis is as far as possible kept at that particular grade of
intensity. The area of cortex cerebri previously ascertained
to produce under faradization extension of the elbow-joint . or
hip-joint is then excited.

For clearness of description let us suppose the left hemi-
sphere excited, and the limb affected the right. The result
of excitation of the appropriate focus in the cortex, e. g.
that presiding over extension of the elbow, is an immediate
relaxation of the biceps with active contraction of the tri-
ceps. As regards the condition of the biceps, the relaxa-
tion is usually so striking that merely to place the finger on it
is enough to convince the observer that the muscle relaxes.
The following is however a good mode of studying the phe-
nomenon: in a monkey with strongly developed musculature
the fore arm, maintained by the above-mentioned steady tonic
flexion at an angle with the upper arm of somewhat less than

282 REACTIONS OF THE MOTOR CORTEX [Lect.

90° is lightly supported by the one hand of the observer,
while with the finger and thumb of the other the belly of the
contracted biceps is felt through the skin. On exciting the
cortex the contracted mass becomes suddenly soft, melting
under the observer's touch. At the same time the observer's
hand supporting the animal's fore arm tends to be pushed down
with a force unmistakably greater than that which the mere
weight of the limb would exert. If the triceps itself be felt at
this time, it is easy to perceive that it enters contraction, becom-
ing increasingly hard and tense, even when its points of attach-
ment are allowed to approximate, and the passive tensile
strain in it should lessen. If the limb be left unsupported the
movement is one of simple extension at the elbow-joint On
discontinuing the excitation of the cortex the fore arm usually
immediately, or almost immediately, returns to its previous
posture of flexion, which is again as before steadily maintained.
Conversely, when, as not unfrequently occurs in conditions
of narcosis resembling that above referred to, the arm has
assumed a posture of extension and this is tonic and maintained,
the opportunity may be taken to excite the appropriate focus
in the cortex for flexion of fore arm or upper arm. Triceps
is then found to relax, and biceps at the same time to enter
into active contraction. If the biceps be hindered from actually
moving the arm, the prominence at the back of the upper arm
due to the contracted triceps is seen simply to sink down and
become flattened. When examined by palpation the muscle is
felt to become more or less suddenly soft, and the biceps at the
same time to become more tense than before. The move-
ment of the limb, when allowed to proceed unhindered, is
flexion with some supination. It is noteworthy that in this
experiment not every part of the large triceps mass becomes
relaxed ; a part of the muscle which extends from the humerus
to the scapula does not in this experiment relax with the rest
of the muscle. This part, if the scapula be fixed, acts as a
retractor of the upper arm, and is not necessarily an antag-
onist of the flexors of the elbow. This part of the triceps we
observed sometimes enter active contraction at the same time

VIII] INHIBITION FROM THE CORTEX 283

as the flexors of the elbow. Under use of currents of moderate
intensity we found that not from one and the same spot in the
cortex can relaxation and contraction of a given muscle be
evoked at different times, but that the tu'o effects are provocable
at different, sometimes widely separate, points of the cortex,
and are there found regularly.

We obtained analogous results in the muscles acting at the
hip-joint In the narcotized animal the hip-joint is being main-
tained in flexion, the thighs being drawn up on the trunk,
excitation of the region of the cortex previously ascertained,
when the limbs hang slack, to evoke extension of the hip, pro-
duces relaxation of the flexors of the hip and at the same time
active contraction of the extensors of the thigh. We examined
particularly the psoas-iltacus, and the tensor fascia femoris^
also the short and long adductor muscles. Each of these was
found to relax under appropriate cortical excitation. If the
knee were held by the observer it was found at the time of
relaxation of the flexors of the hip to be forced downward by
active extension of the hip.

Similarly with other groups of antagonistic muscles, both
those of the small apical joints of the limb, e. g. flexors and
extensors of the digits, and those of the large proximal joints,
e,g, adductors and abductors of the shoulder. At these also
instances of reciprocal innervation were obtained. By antagonis-
tic muscles I mean only what are termed true antagonistics ; I do
not include the cases where one muscle Axes a joint enabling
another muscle to thus act better on another joint — H. E. Ber-
ing's pseudoantagonists.^^ Hering has carefully analyzed ^*
the co-ordination of such pseudoantagonists in the action of
clenching the flst He has shown that when that movement is
evoked in the monkey by excitation of the cortex cerebri the ex-
tensors of the wrist are thrown into action simultaneously with
the long flexor of the fingers. But there was no evidence
that the true antagonists were ever thrown into simultaneous
activity.

That a part of the triceps brachii (that retracting the upper
arm) should actively contract exactly when another part (that ex-

284 REACTIONS OF THE MOTOR CORTEX [Lect.

tending the elbow) becomes relaxed is exactly comparable with
a phenomenon which can be noted in the limb under spinal re-
flexes, both in triceps itself and in quadriceps femaris. Beevor and
others have shown that different parts of what in gross anatomy
is denominated one single muscle are used separately in various
movements. And Hering and I similarly saw in the quadriceps
femoris, on exciting the cortical region yielding extension of the
hip, a relaxation of a part of the quadriceps (a part which flexes
the hip) with contraction of another part (which extends the
knee). I have also noted that in the monkey by stimulating
the appropriate cortical area for flexion of the knee the knee-
jerk is temporarily depressed or suppressed completely.

The results obtained from the internal capsule were as strik-
ing as those obtained from the cortex itself. From separate points
of the cross-section of the capsula, relaxation of various muscles
was evoked. Among the muscles whose inhibition was directly
observed were supinator longus and biceps brachii^ the triceps^ the
deltoid^ the extensor cruris^ the hamstring group, the flexor
muscles of the ankle-joint, and the stemomastoid.

The spots in the cross-section of the capsula which yielded
the inhibitions were constant, that is, the position of each when
observed remained constant throughout the experiment The
area of the capsular cross-section at which the inhibition of the
activity of, e, g, the triceps, muscle can be evoked is separate
from (that is to say not the same as) that area whence excitation
evokes contraction of the triceps (or of that part of the triceps
inhibition of which is now referred to). On the other hand, the
area of the section of the internal capsule, whence inhibition of
the muscle is elicited, corresponds with the area whence con-
traction of its antagonistic muscles can be evoked. Yet synchro-
nous contraction of such pairs of muscles as gastrocnemius and
peroneus longus is obtainable from the cortex. The observa-
tions make it clear that " reciprocal innervation " in antagonistic
muscles is obtainable by excitation of the fibres of the internal
capsule. Topolanski ^®* observed them on exciting the corpora
quadrigetnina (rabbit). It is probable therefore that the in-
hibition elicitable from the cortex cerebri is not in these cases

VIII] INHIBITION PROBABLY SUBCORTICAL 285

chiefly or at all due to an interaction of cortical neurones one
with another.

Exner drew from his observations on the rabbit a similar in-
ference that the inhibitory phenomena had their chief seat in the
spinal mechanisms though elicited from the cortex. My own
inference has been that the seat of inhibition in these reactions
from the *' motor " cortex lies probably at the place of confluence
of conducting channels in a common path, likely enough at their
confluence upon the ** final common path^' the motor neurone,
that is at the ultimate synapse. But it may well be, indeed is in
the highest degree likely, that in other fields of action one corti-
cal element inhibits another cortical element

In ^^ willed'' movements of the eyeballs of the monkey the same
kind of co-ordination was revealed in some observations I ob-
tained on this point When the III and IV cranial nerves had
been resected intracranially and the animals in ten days or so
had recovered completely from the surgical interference, the eye
movements were examined.

In these animals if the gaze was attracted to an object, e. g.
food, held level with the eyes and to the right of the median
plane (left III and IV nerves cut) the left eye looked straight
forward, the right looked to the right. If the object was then
shifted more to the right or less to the right, the right eye fol-
lowed it, moving as the object was moved, while the left eye
remained motionless, looking straight forward all the time.
But when the object was held to the left of the median plane
both eyes were directed upon it, apparently quite accurately.
When the object was shifted farther and farther to the left both
eyes followed it with a steady conjugate movement not detectably
different from the normal. When the object was carried from
the left-hand verge of the field back toward the median plane
both eyes followed it as accurately as before. If the object was
moved suddenly fi'om the extreme left-hand edge of the field up
to the median plane both eyes immediately and apparently
equally quickly reverted to parallelism with that plane. Or, if
the object were suddenly brought back from the left edge of
the visual field to some point intermediate between that and the

286 REACTIONS OF THE MOTOR CORTEX [Lect.

median plane both eyes at once shifted apparently equally to a
correspondingly diminished deviation from the primary posi-
tion. These actions must mean that in the left eye relaxation of
^rectus extemus kept accurate time and step with contraction of
recttis extemus of right eye. And the action of the left rectus
extemus gives presumably a faithful picture of a synchronous
process going forward in the right rectus intemus.

It is interesting to recall that in the seventeenth century
Descartes in his De Homine,* discussing willed movements,

Figure 74. — A. Figure from the De Homint of Descartes, edit, of i66a*, in which he illus-
trates his conception of the co-ordination of the antagonistic muscles of the eyeball by the
above drawing from his own hand.

B. Figure illustrating the same text in De Homine, edit, of 1677^; in this the
sketch by Descartes has been much elaborated.

suggested for the mechanism of the lateral movements of the
eyeball, with which he deals in some detail, a co-ordination
much resembling reciprocal innervation. He urged that the
vital spirits were conducted into the external rectus by valved
channels in that muscle, and at the same time were from the
internal rectus led out by valved channels, so that as the one

VIII] RECIPROCAL INHIBITION 287

muscle became tense by distension the other became flaccid by
emptying. He furnished with his own pencil a figure illus-
trating the mechanism as he conceived it (Fig. 74). There is
an essential resemblance between his scheme and that of " recip-
rocal innervation/' except that he imagined the mechanism a
peripheral one, that is to say that the ** inhibition/' as we now
term it, had its seat in the muscle, not in the nerve-centres
themselves.

Again, early last century (1823) Charles Bell, in a footnote to
a paper in the Philosophical Transactions,^ argued a similar kind
of co-ordinate mechanism in the execution of willed movements.
He wrote : " The nerves have been considered so generally as
instruments for stimulating the muscles, without thought of their
acting in the opposite capacity, that some additional illustration
may be necessary here. Through the nerves is established the
connection between the muscles, not only that connection by
which muscles combine to one effort but also that relation be-
tween the classes of muscles by which the one relaxes and the
other contracts. I appended a weight to a tendon of an ex-
tensor muscle which gently stretched it and drew out the muscle ;
and I found that the contraction of the opponent flexor was
attended with a descent of the weight, which indicated the relaxa-
tion of the extensor/' " If such a relationship be established,
through the distribution of the nerves, between the muscles of
the eyelids and the superior oblique muscles of the eyeball, the
one will relax while the other contracts." But like Descartes he
pictured a peripheral inhibition, for he says : ** If we suppose
that the influence of the 4th nerve is, on certain occasions, to
cause a relaxation of the muscle to which it goes, the eyeball
must be then rolled upwards." Descartes and Bell, therefore,
with remarkable prescience imagined the existence of an action
of nerve on muscle just such as was later actually discovered by
the Webers*^ in the vagal inhibition of the heart — an inhibition
which Volkmann ^' previous to the Webers met in the course of
experiment, but unevpcctant of it, rejected ^® as illusory and due
to some experimental error.

288 REACTIONS OF THE MOTOR CORTEX [Lect.

As for salient objective differences observable between move-
ments elicited from the so-called " motor " cortex and those
of spinal reflexes, these are for the most part less clear than
might at first be supposed. The general statement that the co-
ordination is of a ** higher " kind in the former has doubtless
truth, but it proves vague when details are demanded. A co-
ordination though simple may yet be perfect. The co-ordination
by which the leg is drawn up in the spinal •* flexion-reflex " seems
as perfect as when the limb is drawn up by stimulation of the
cortex. It is true that in the dog's scratching movement elicited
as a spinal reflex after transection of the cord the foot in my
experience practically never attains accurately the site of the
stimulation, although broadly directed toward it. Were the
movement elicited from the cortex one would expect it to be
more accurate in this respect. But I have never succeeded in
eliciting this action from the cortex, and so am unable to insti-
tute the comparison.

If by higher co-ordination it be meant that larger groups of
reflex systems are simultaneously thrown into — or out of —
action under cortical excitation than in merely spinal reflexes,
that is more than probable. Yet we must admit that the field
of musculature thrown into action by a focal stimulation of the
cortex seems in some cases extremely limited. Thus A. S.
Griinbaum and myself have seen that in the chimpanzee and
gorilla any single manual digit can be moved isolatedly by
stimulation of the cortex. We must not forget, however, that
with even a small movement the field of inhibition may yet be
wide, for I have on occasion noted inhibition of muscles of the
shoulder when the thumb was moved under cortical excitation,
the shoulder previously being unrelaxed.

There is the well-known clonic after- discharge following
cortical stimulation. But a marked after-discharge is also
usual in spinal reflexes, rhythmic in rhythmic reflexes, tetano-
clonic in tetanic reflexes e. g, in the " flexion-reflex," and is
sometimes enormously prolonged (Figs. 49 and 57). So that
even this difference is less marked than is customarily thought.
Certain other differences appear to me more significant When

VIII] CORTICAL AND SPINAL REACTIONS 289

a spinal reflex is prolonged under strong stimulation its dis-
charge spreads, developing what Dr. Hughlings Jackson ^ has
termed a " march." The " march " of the spinal reaction tends
to transgress the median line more than does that of a cortical
reaction, which tends rather to spread unilaterally. Also the
progress of the spinal " march " runs a more rapid course than
does the cortical.

In the synthesis of movement in the animal it is obvious
that these reactions elicitable from the motor cortex fall into
three groups, like the three groups above distinguished in
spinal reflexes (Lect. IV). In one group the movement evoked
from the cortex of one hemisphere seems a fraction of a natural
movement, the natural movement requiring in its completeness
the co-operation of the symmetrical area of the cortex of the
opposite hemisphere. Opening of the jaw as elicited from one
hemisphere, e. g. the left, is seen after the jaw is split at the
symphysis to be executed by the muscles of the crossed, 1. e.
right, half of the jaw, the muscles of the left half being very
slightly activated or not at all. This cortical movement is
evidently incQmplete and fractional. The inference is unavoid-
able that in the natural undeviated opening of the mouth the
actions of symmetrical areas of the right and left cortices are
coupled as " allied'* reactions. In a second group, instanced by
conjugate lateral deviation of the eyeballs toward the opposite side,
it is equally obvious that the reactions of symmetrical areas of
the right and left cortices are related one to another as " antago-
nistic " reactions. Such reactions have inhibitory relation one
to another (151, 304). They must have this inhibitory relation
even when combined, as Mott and Schafer showed they can be,
to yield convergence of the ocular axes under bilateral excita-
tion of the right and left hemispheres. In a third group of
cases the reactions of symmetrical cortical areas right and left
seem neutral one to another. Thus, with the area which yields
movement of the thumb that reaction seems neither to reinforce
nor to interfere with the similar reaction evoked from the twin
area of the opposite hemisphere. That the reactions are really
wholly neutral one to another is of course diflicult to say

290 REACTIONS OF THE MOTOR CORTEX [Lect.

because the experimental observations are carried out under
narcosis, and the narcosis probably sets in abeyance many co-
ordinating mechanisms of the brain itself. It is clear, however,
that the same broad groups of interrelationship (alliance, inter-
ference, neutrality) as were traced in bulbospinal reflexes re-
appear also between motor reactions of symmetrical areas of
the cortex of the hemispheres.

It is striking that the complete, i. e. perfectly balanced,
bilaterality of motor representation with which the Broca motor
speech centre is credited, no doubt justly, is exceptional in
the motor cortex. Beevor and Horsley pointed out that move-
ments of perfectly balanced bilaterality are of much rarer dis-
tribution in the cortex of the hemisphere than was generally
supposed. I incline to think that even the small category of
such movements which they admit will have to be reduced
further by the removal from it of " mastication." Certain it is
that for a group of movements to be perfectly bilaterally repre-
sented in an area of one hemisphere and not equally in the
corresponding area of the other hemisphere, the state of things
generally supposed for the Broca centre, is an arrangement
wholly unknown in the motor cortex. It shows how different
must be the operations of the Broca area from those of the
areas of the so called motor cortex. Griinbaum and myself in
excitation experiments could get no evidence of a Broca centre
in the anthropoid apes.

One broad resemblance between the movements elicited
from the motor cortex and spinal reflexes is striking, and yet
is, I think, not insisted on by writers. We have seen that the
movements elicitable in the various regions as local reflexes
by stimulation of the afferent paths of those regions present
regular and characteristic quality. Thus stimuli to a fore limb
induce lifting of that limb with flexion at elbow and retraction-
flexion at shoulder; stimuli to a hind limb induce drawing up
of that limb with flexion at knee, hip, and dorsi-flexion at
ankle; stimuli to the mouth induce opening of the jaws, and
so on. While these movements are emphatically evidenced as
local spinal reactions the overwhelming predominance of their

VIII] RELATIONS BETWEEN MOVEMENTS 291

occurrence equally emphasizes the scarcity of occurrence of
certain other movements as local spinal reactions. Extension
of the hind limb can, it is true, be evoked from that limb
as a spinal reflex by a certain special form of stimulus, but
that stimulus has to be applied to a special part of the foot
and is successful only after ** spinal shock " has passed off;
while the flexion-reflex can be evoked by various forms of
stimuli applied practically to any point of the limb surface, and
is elicitable almost from the very hour of spinal transection
onward. So also I have occasionally succeeded in evoking
closure instead of opening of the jaw by stimulation of a certain
part of the lip in the decerebrate animal — but even then the
reflex is not regularly elicitable. On the other hand reflex
opening of the jaws is easily and regularly elicitable from
various points of the oral surface. Similarly extension of the
elbow as a local reflex elicitable by stimuli applied to the fore
limb itself is a reflex practically unknown to me.*

Now those movements that are practically wanting as local
bulbo-spinal reflexes and strike the observer of the spinal or
decerebrate animal by their default, are likewise practically
absent, comparatively infrequent, or only limitedly and irregu-
larly elicitable from the motor cortex itself.*^ On the other
hand, the movements regularly and widely elicitable as local
reflexes are liberally represented in the motor cortex.**

In the light of the observations mentioned above, which show
that reciprocal innervation is a mode of co-ordination widely
exhibited in the reactions elicitable through the motor cortex,
this sparse occurrence of certain movements, e. g, extension of
the knee or closure of the jaw, does not mean that the ex-
tensor muscles of the knee or the muscles which close the
jaws are unrepresented in the cortex. It does not mean that
this cortex is in touch with the flexors alone and not with the
extensors. It means that the usual effect of the cortex on these
latter is inhibition. It means not that the extensors and the

* Extension of elbow and of knee are of course easily obtainable as crossed
reflexes and as parts of reflexes evoked from distant points. Sach reflexes I do not
include as iceal reflex reactions.

292 REACTIONS OF THE MOTOR CORTEX [Lect.

jaw-closers are unrepresented cortically, but that their normal
representation in the cortex under the ordinary conditions of
experiment has the form of inhibition, not excitation, and thus
unless specially sought escapes observation.

Since, as above shown, strychnine and tetanus toxin trans-
form certain inhibitions into excitations, we have a means of
further testing this point It is in my experience quite ex-
ceptional to obtain primary extension of the opposite knee
as a motor reaction from the cerebral cortex of the cat — or
even, indeed, as a secondary movement. In exploring the
cortex with unipolar faradization I have often failed to elicit
the movement at all throughout a series of observations.
Flexion, on the other hand, is regularly obtainable. After
exhibition of strychnine the extension of knee can be regu-
larly excited from the cortex, and from the very points of it
that yielded flexion previously. This conversion is not so facile
as the conversion of the spinal reflex. The dose of strych-
nine has to be larger, or to operate longer. With doses addi-
tively given, there seems, early in the experiment, a period when
reflex spinal inhibition of the extensors has been converted into
excitation, but the cortex of the brain still yields knee-flexion,
not knee-extension. The cortical reversal has required in my
hands doses that evoke convulsive seizures from time to time.
I have seen, immediately after a severe convulsion, the cortex
either unable to evoke any movement of the knee or produce
knee-flexion, though a short while before it gave knee-extension.

Tetanus toxin likewise converts the cortical flexion into
extension. The efTect is in its case the more marked, because,
if the cortical examination be at an early stage of the progressive
malady ensuing on inoculation by a moderate dose, or where
the dose has been quite small, the tetanus is " local " and con-
fined to the inoculated limb, and then, if the tetanus be " local "
in one hind limb, e, g, the left, the appropriate area of the right
hemisphere yields knee-extension, whereas the corresponding of
the left hemisphere yields knee-flexion.

But these effects are better studied in the monkey. There,
in my experience, to obtain primary extension of the crossed

VIII] REPRESENTATION OF MOVEMENTS 293

knee from the cortex is, as in the cat, extremely unusual. A
number of experiments can be made without obtaining it at alL
Even as a secondary movement it is extremely poorly repre-
sented in the cortex. For twenty instances of flexion at knee
it is, in my experience, often difficult to find one of extension at
that joint But after tetanus toxin or strychnine the whole " leg-
area " of the cortex, from all points of its surface, may yield
nothing but leg-extension, in which extension of knee is prominent
as an evident part of a primary combined movement This is espe-
cially striking when the tetanus is still merely " local," and con-
fined to one hind limb, e. g. lefL The " leg-area " of the right
cortex then yields knee-extension everywhere ; the " leg-area "
of the left cortex yields the normal flexion results. The " leg-
area " of the right cortex provokes moreover from many of its
points extension of right knee and ankle, as well as of left, though
less strongly. The " leg-area " of left hemisphere does this
little, if at all. Under moderate faradization the "leg-area" in
the monkey, in my experience, moves the homonymous hind
limb, in addition to the crossed, very slightly and rarely, much
less easily than in the cat, though in both the movement is the
same, namely, "extension." So localized may be the toxic
influence in its early stage that reversal of the usual cortical
effect at knee may obtain while in the same hemisphere that on
hip and ankle still remain flexion as usual.

Similarly with the " arm-area." In the cat, it is in my expe-
rience quite infrequent to obtain primary extension of the crossed
elbow from the cortex. Flexion is readily and regularly obtained.
Strychnine changes this : the very surface that yielded flexion
then provokes extension, and strongly. But the dose of strych-
nine seems to be larger than for conversion of the spinal reflex,
and the conversion shows the phases before mentioned in regard
to the knee-inhibition, and its conversion in the case of the ham-
string nerve. In the monkey, in my experience, the effect of
strychnine and of tetanus toxin when pushed to the general
convulsive stage is often contrary to the effect in that stage in
so many other animals. I have seen them, though producing
extension at elbow at first, later produce flexion at elbow.

294 REACTIONS OF THE MOTOR CORTEX [Lect.

In one case» in a monkey, in which the tetanus had become
general in the sense that only one limb was unaffected, the
affected arm was strongly extended and rigid at elbow with
some retraction at shoulder. But in all my instances, where by
introduction of the toxin into the trunk of the median or ulnar a
" local " tetanus of the arm has been produced, the limb has been
extended rigidly at elbow and retracted at shoulder. In these
cases faradic examination of the cortex showed that the small
field of the " arm-area" to which extension at elbow is restricted,
was enlarged so as to include the whole " arm-area." Under
the toxin the cortex that normally in the cat yields flexion
of the crossed fore limb and extension of the uncrossed, will
-yield extension of both when there is local tetanus in the
crossed limb. Extension at elbow sometimes alone, more
often with retraction at shoulder, or with extension at wrist
or fingers, sometimes as a leading movement, sometimes
rapidly ensuent on retraction at shoulder or extension in the
hand, according as higher or lower points in the area were
stimulated, was prominently exhibited at all points of the entire
surface of the " arm-area/' That area, with this as its salient
reaction, seemed particularly in evidence, for its extreme limits
appeared traceable further than usual, and to encroach on or
overlap more than is usual under the feeble or moderate stimu-
lation employed, the *' leg-area " above and the " face-area "
below, and to run exceptionally far forward above the pre-
central sulcus, though remaining undemonstrable in the free
surface of the ascending parietal convolution. From no point
in all this extensive " arm-area " was, despite repeated trials,
any flexion at elbow or shoulder or hand obtained (Fig. 75).
Various intensities of faradization were employed, and points
known normally to yield it most regularly were tried : but ex-
tension, not flexion, always resulted.

This condition of the " arm-area " can in tetanus exist in one
hemisphere or even in both hemispheres and the ** leg-area " of
each hemisphere yet yield flexions at knee and hip and ankle,
and its other normal forms of reaction. Tetanus produced
by introduction of the toxin into the arm (^. ^. median or

VIII] STRYCHNINE AND THE CORTEX 295

ulnar trunk) affects subsequently to the inoculated limb, the
fellow fore limb first, and the jaw before the hind limbs, although
the knee-jerk on the homonymous side to the inoculation may
be brisk.

Under decerebrate rigidity, e. g. in the cat, the closing
muscles of the jaw are kept in tonic action, holding the mouth
somewhat shut^® By stimulation of any point of a large " skin-
area " appropriate for the reflex, reflex opening of the mouth,
including depression of the lower jaw, is easily and regfularly
elicited, or by faradization of an afferent twig of the trigeminus;
or as was shown by Woodworth and myself,^* even by stimula-
tion of distant afferent nerves, e. g. plantar or saphenous. Here
the action of the powerful closing muscles is reflexly inhibited
while the weaker opening muscles are reflexly excited — it
seems, in fact, a case of Astacus claw, except that the inhibi-
tion is central, not peripheral. This reflex " opening " is in
the decerebrate animal converted into reflex closure by tetanus
toxin and by strychnine, the inhibition of the predominantly
powerful closing muscles being converted into excitation of
them.

Similarly, when the " face-area " of the monkey's cortex is
tested by faradization after exhibition of strychnine the points
of surface that previously yielded regularly the free opening of
the jaw, yield strong closure of the jaw instead. Now closure of
the jaw is a movement of very limited representation in the cor-
tex of the monkey, even of the anthropoid. On the other hand,
opening of the jaw is always readily and regularly elicitable from
a large field of the " face-area." And adjoining and overlapping
this large area whence steady opening of the jaw is obtained, is
found an area whence, as Ferrier *® first pointed out, " rhythmic
alternating opening and closing of the jaws," as in feeding, can
be evoked. Under tetanus toxin (Fig. 75) and strychnine the
whole of this combined area not only ceases to yield opening of
the jaws, either maintained or rhythmic, but yields closing of
them instead — often with visible retraction of the tongue. For
this conversion larger doses of strychnine have, in my hands,
been required than for conversion of knee-flexion into extension.

:395 Ig^TALiaUB^ OF THE MOTOR CORTEX [Lect.

ITmi TesBoaS' mcin "fbt vanvcrAan Jtpp e ms tiie more striking
-v^tcD eEBmined carhriii list progress of liie TntnYiration, because
i! nxBT br :KiimQ ai a stage prmffdrng mhcgctbcr the ocxurrence
rd" VD} general cciiin]}aiai&, «nd aOso because it can then some-
limeE be fninid id be ■rnnTalrral, lliat is, to be present in the
** &ce^«rca " of cme Itemffiphe re * indiont or almost without any
acnecrinn nf the '**&ce^am'* of tte other hcmi^here. The
rranriaag rf "fbt snrmal £efld '&a& rrmmm far comparison in the
same xnOTridcal 'wnfb. ibe readScms of the aboormal field.

Trtaniff toxixi sfaovs ffnarkrri predilectioa for the closure
mechanism of the jaw. A&cr iaiocnlalSon in the hind leg, e\'en
bcfuffT ihe '^iDcal'^ trTarnis has obvk>asly invaded the feUow
Hiob of ibe DpposBte side, a slight tightness of jaw and an
rmrn c^bTie psrsng of ihe £ps has several times given warning
liha: gczieral teianizs had rcal}j set in, before any trace of
ge aer a ] comralsTe seiznies or any involvement of the arms
was dciecied. Tetanizs toxin has also certainly intensified the
reactions of the cortical areas that give retraction of the neck
and retraction of the abdominal wall (Fig. 75).

The progress of the change wrought by these agents in con-
i-erdng these reactions of the cortex from their usual form to
the diametrically opposed seems to in\*ol\'e the same kind of
steps as that noted above in their conversion of the inhibitor}'
ha:::string nerre enect on the knee-extensor. Stages can be
found in which the inhibitor}- effect is less than normal, yet is
not replaced by excitator>\ With the cortical opening of jaw,
in early tetanus a grade is discoverable when faradization of the
cortex produces a slight opening of the jaw — a mere *• loosen-
ing" of the jaws, so to say — distinctly less than normal, and
hardly effectively opening the mouth. Also ^^-ith the ** leg-area "
of the cortex, at an early stage of the tetanus it would seem that
an undue but far from exclusive preponderance of plantar exten-
sion at ankle over dorsal flexion at that joint exists, while the
symptomatic knee-extension is as yet not excitable though knee-

• The hemisphere, the ** face-area " of which is earlier affected, is, in the case of
jnoculation in a limb, the hemisphere contralateral to the limb inoculated.

VIII] CORTICAL ANTAGONISTIC MOVEMENTS 297

Body

LM%

ARM ^r

FACfc

NfCK

ARM ^z2

FACE

Fig. 75. — Outline to illastnite the changes produced bj tetanus-toxin in the functional topog-
raphy of the motor-cortex of the monkey, CercofUhtcus ealiitkrix, CF = the central
fissure. F mdicates hip-knee flexion, E indicates hip4mee extension ; / indicates elbow
flexion, £ indicates elbow extension ; < indicates jaw opening, = indicates jaw closing ;
prosthotonic indicates in regard to ** body " ventral bending, opisthotonic indicates dorsal
bending. The distribution of these symbols in the drawing indicates broadly the field
whence could be elicited the movements that the symbols respectively stand for, in A
before and in B after development of lockjaw. In B at the lowest part of the face^rea a
place still yielded opening of the mouth. In the experiment which furnished the specimen
figured the site of inoculation had been the 1^, hence the toxtc actfcMi reached flte jaw
comparatively late. Had the exploration of the cortex beeftdeferred even longer the open
big of the jaw might perhaps have been transformed to closure tfan>ughout the cortex.

298 REACTIONJS OF THE MOTOR CORTEX [Lect.

flexion is almost in abeyance. Neither under tetanus toxin or
strychnine have I at present observed conversion of the abducens
inhibition into excitation.

The foregoing observations appear to give an insight into at
least a part of the essential nature of the condition brought
about by tetanus and by strychnine poisoning. These disorders
work havoc with the co-ordinating mechanisms of the central
nervous system because in regard to certain great groups of
musculature they change the reciprocal inhibitions, normally as-
sured by the central nervous mechanisms, into excitations. The
sufferer is subjected to a disorder of co-ordination which, though
not necessarily of itself accompanied by physical pain, inflicts
on the mind, which still remains clear, a disability inexpressibly
distressing. Each attempt to execute certain muscular acts of
vital importance, such as the taking of food, is defeated because
from the attempt results an act exactly the opposite to that
intended. The endeavour to open the jaw to take food or drink
induces closure of the jaw, because the normal inhibition of the
stronger set of muscles — the closing muscles — is by the agent
converted into excitation of them. Moreover, the various reflex-
arcs that cause inhibition of these muscles not only cause exci-
tation of them instead, but are, periodically or more or less
constantly, in a state of super-excitement, and yet attempt on
the part of the sufferer to restrain, to inhibit, their reflex reaction,
instead of relaxing them, only heightens their excitation further,
and thus exacerbates a rigidity or a convulsion already in
progress.

It seems to me not improbable that the virus of rabies may
similarly upset reciprocal innervation, though its field of opera-
tion, at least in man, lies not in the same group of mechanisms
as are affected by strychnine and tetanus toxin but in an
allied one, namely, that inter-regulating (by co-ordinations in-
volving inhibition, as Meltzer and Kronecker showed) the acts
of deglutition and respiration.

Little has met me in the course of observations on the re-
actions of the cortex under strychnine or tetanus toxin to indi-
cate that the transformation of the motor effects of the reactions

VIII] LOCKJAW 299

is due to action of these agents on the cortex itself. The change
of result seems quite explicable by alteration produced in lower
centres, e, g. spinal and bulbar, on which the cortex acts. This
seems especially shown by the toxin when injected into the right
arm and producing extensor rigidity at that elbow and rigid
torticollis to the right, converting the flexion of arm-area of the
left hemisphere into extension under arm-area excitation, and
in the right hemisphere torticollis movement to the right

The vast rdle of inhibition in cerebral processes as evidenced
by mental reactions, and the slightness of mental disorder in
strychnine poisoning or tetanus indicates a difference between
inhibition as it occurs in the bulbo-spinal arcs and in the arcs
of purely sensual and perceptual level, a difference presumably
of physicochemical nature.

We find, therefore, these reactions changed in a like manner
by strychnine and tetanus whether we excite them from the
cortex or from reflex spinal arcs. And a further similarity be-
tween the representation of movement in the motor cortex and
in the bulbo-spinal axis as a mechanism for local reflexes is the
following. When the induced movement embraces both hind
limbs or both fore limbs it is in an opposite sense in the two
limbs. Thus the crossed accompaniment to the flexion-reflex
of the limb is extension : and so also when cortical stimulation
evokes {e.g. in cat) flexion e.g. of the right fore limb, not rarely
it evokes movement, weaker it is true, in the left, and that move-
ment, as Exner" noted in the rabbit, is extension.

The local reflex movements obtainable from the bulbo-spinal
animal and the reactions elicitable from the motor cortex of the
narcotized animal fall into line as similar series. Both consist
of the same group. But in striking contrast to this group stands
the motor innervation active in " decerebrate rigidity."

Decerebrate rigidity ^^ is a condition which ensues on remo-
val of the fore-brain by transection at any of the various levels
in the mesencephalon or the thalamencephalon in its hinder part

If in a monkey or cat transection below or in the lower half
of the bulb has been performed, the animal when suspended.

300 REACTIONS OF THE MOTOR CORTEX [Lect.

artificial respiration if necessary being kept up, hangs from the
suspension points with deeply drooped neck» deeply drooped
tail, and its pendent limbs flaccid and slightly flexed. The fore
limb is slightly flexed at shoulder, at elbow, and very slighdy
at wrist The hind limb is slightly flexed at hip, at knee, and
at ankle. On giving the hand or foot a push forward and then
releasing it, the limb swings back into and somewhat beyond the
position of its equilibrium under gravity; and it oscillates a few
times backward and forward before finally settling down to its
original position.

To this condition of flaccid paralysis supervening upon tran-
section in the lower half of the bulb the condition ensuing on
removal of the cerebral hemispheres offers a great contrast. In
the latter case the animal, on being suspended just as after the
former operation, hangs with its fore limbs thrust backward, with
retraction at shoulder joint, straightened elbow, and some flexion
at wrist. The hand of the monkey is turned with its palmar
face somewhat inward. The hind limbs are similarly straight-
ened and thrust backward ; the hip is extended, the knee very
stiffly extended, and the ankle somewhat extended. The tail
in spite of its own weight, and it is quite heavy in some species
of monkey, is kept either straight and horizontal or often stiffly
curved upward. There is a little opisthotonus of the lumbo-
sacral vertebral region. The head is kept lifted against gravity
and the chin is tilted upward under the retraction and backward
rotation of the skull on the neck. The mouth is kept closed
and there is some stiffness in the elevators of the jaw. When
the limbs or tail or head or jaw are pushed from the pose they
have assumed considerable resistance to the movement is felt,
and unlike the condition afler bulbar section, on being released
they spring back at once to their former position and remain
there for a time more rigid than before.

The rigidity is immediately due to prolonged spasm of cer-
tain groups of voluntary muscles. The chief of these are the
retractor muscles of the head and neck, the elevators of the jaw
and tail, and the extensor muscles of the elbow and knee, and
shoulder {i,e, deltoids) and hip. In the dog and cat, just as

VIII] DECEREBRATE RIGIDITY 301

spinal shock is more severe in the fore limbs than in the hind, so
decerebrate rigidity is more marked in the fore than in the
hind limb. This prolonged spasm may be maintained, with
intermissions, for a period of four days. It is increased, and
even when absent or very slight, may be soon developed by
passive movements of the part There is no obvious tremor
in the spasm in the earlier hours of its continuance; later it
does sometimes become tremulant.

Administration of chloroform and ether, if carried far, quite
abolishes the rigidity. On interrupting the administration the
rigidity again rapidly returns.

Section of the dorsal columns of the spinal cord does not
abolish the rigidity. Section of one lateral column of the cord
in the upper lumbar region abolishes the rigidity in the hind
limb of the same side as the section. Section of one ventro-
lateral column of the cord in the cervical region destroys the
rigidity in the fore and hind limbs of the same side.

The rigidity develops either very imperfectly or not at all
in a limb the afferent roots of which have been severed some
days prior to carrying out the operation which produces the
rigidity.

If after ablation of both cerebral hemispheres, even when
the rigidity is being maintained at its extreme height, the affer-
ent roots previously laid bare and prepared, are carefully severed,
the limb at once falls flaccid. The result is quite local, that is,
confined to the one limb the afferent roots of which are severed.

Decerebrate rigidity exhibits reflex excitation in those very
groups of muscles which the local reflexes and the motor cortex
when stimulated excite but little. Not that the muscles exhibit-
ing the rigidity are absolutely unamenable to transient spinal
reflexes. The extensor-thrust and certain crossed reflexes are
witness to the contrary. And they are not absolutely unamen>
able to cortical excitation. The extension of the elbow obtain-
able from the cortex refutes that But these instances do not
efface the broad fact that a wide system of musculature, includ-
ing the extensors of the hip, knee, shoulder, and elbow, and
the elevators of the tail, neck, and jaw, is inhibited by the

302 REACTIONS OF THE MOTOR CORTEX [Lect.

overwhelming majority of local spinal reflexes and of reactions
from the motor cortex, but on the contrary is excited in a set
of reflex reactions which employ the local deep aflerents (pro-
prioceptive) and some cranial mechanism seated between cere-
brum and bulb. The cerebellum at once rises to mind. But I
found ablation of the cerebellum did not abolish the rigidity.
It is signiflcant that a vertical posture favors the appearance
and development of the rigidity. The muscles it predominandy
aflects are those which in that attitude antagonize gravity.
In standing, walking, running, the limbs would sink under the
body's weight but for contraction of the extensors of hip, knee,
ankle, shoulder, elbow; the head would hang but for the re-
tractors of the neck ; the tail and jaw would drop but for their
elevator muscles. These muscles counteract a force (gravity)
that continually threatens to upset the natural posture. The
force acts continuously and the muscles exhibit continued action,
tonus. We seem to have here a fleld of muscles combined as
a physiological entity. A characteristic reaction yielded by
muscles of this field is the " jerk," the " tendon phenomenon,"
itself a sign of highly maintained reflex tonus.

Two separable systems of motor innervation appear thus
controlling two sets of musculature: one system exhibits
those transient phases of heightened reaction which constitute
reflex movements; the other maintains that steady tonic re-
sponse which supplies the muscular tension necessary to
attitude. Starting from the tonic innervation as initial state
the first step in movement tends to be flexion and involves
under " reciprocal innervation " an inhibition of the extensor
excitation then in process. This will be involved whether the
excitation be via local reflex or via the motor cortex. Hence
the very muscles that to the observer are most obviously under
excitation by the tonic system are those most obviously in-
hibited by the phasic reflex system. And the tonic system
will, on inhibition of it passing off", contribute toward a return
movement to the pre-existing pose, thus having its share in
alternating movements and in compensatory reflexes. These
two systems, the tonic and the phasic reflex systems, co-operate

VIII] DECEREBRATE RIGIDITY 303

exerting influences complemental to each other upon various
units of the musculature. Drugs and other agents that act
in a selective way upon nervous processes might be expected
in some cases to throw into relief the operation of one or
other member of this paired system. Strychnine and tetanus
toxin administered to an animal in decerebrate rigidity increase
that rigidity. The posture assumed by the limbs, neck, tail,
head, etc., in strychnine poisoning and in tetanus resembles
closely in many respects the attitude of decerebrate rigidity.
There are differences, — for instance, the ankle is often rigidly
extended in tetanus whereas it is little affected in decerebrate
rigidity ; nevertheless there is much general resemblance.

And just as certain agents display their action more obvi-
ously in one member of these paired systems than in the other
so processes of disease may be expected to deal with the two
systems unequally and to reveal more obviously and affect
more deeply one of them than the other. Hughlings Jack-
son '•»™»^ with characteristic penetration of thought argued
nearly thirty years ago that rigidity ensuing in hemiplegia
(hemiplegic contracture) is not owing to the cerebral lesion
nor to the lateral sclerosis. He said : " Whilst the primary cere-
bral lesion can account for the paralytic element it cannot (nor
can the sclerosis of the lateral column) account for the tonic
condition of the muscles. My speculation is that the rigidity is
owing to unantagonized influence of the cerebellum. Whilst
the cerebrum innervates the muscles in the order of their action
from the most voluntary movements (limbs) to the most auto-
matic (trunk), the cerebellum innervates them in the opposite
order. This is equivalent to saying that the cerebellum is the
centre for continuous movements and the cerebrum for chang-
ing movements. Thus in 'walking' the cerebellum tends to
stiffen all the muscles ; the changing movements of walking are
the result of cei'ebral discharges overcoming in a particular and
orderly way the otherwise continuous cerebellar influence.
When the influence of the cerebrum is permanently taken off
by disease of the cerebrum, as in hemiplegia, from the parts
which it most specially governs (arm and leg) the cerebellar

3CH REACTIONS OF THE MOTOR CORTEX [Lect.

influence is no longer antagonized; there is unimpeded cere-
bellar influx and hence rigidity of the muscles which in health
the cerebrum chiefly innervates. The spinal muscles are those
which the cerebrum influences least and the cerebellum most
In health the whole of the muscles of the body are doubly
innervated — innervated both by the cerebrum and cerebel-
lum: there being a co-operation of antagonism between the
two great centres."

This view of Hughlings Jackson seems supported and
amplified by Luciani and Stefani's work on the cerebellum.
Wernicke,^** Mann,^*"* and Lewandowski *^ also point out that
cerebral paresis selects one group of antagonistic groups of
muscles in the limbs. We may very likely have to seek in the
aflerent nerves of muscles — especially of those antagonizing
gravity — and in the nerve of the otic labyrinth — the " tonus-
labyrinth " of Ewald — the sources of the influence to which
Hughlings Jackson refers as "cerebellar," but that does not
radically aflect in its main feature the scheme he draws of a
changeful ** clonic " (I would prefer to say " phasic ") innervation
and a relatively unchanging tonic innervation as two systems in
co-operative antagonism. Although we must also admit that
the cortical innervation, pre-eminendy phasic though it be, also
is to some extent tonic; Lewandowski's *^ study of hemiplegic
contracture seems to make this certain.

And here arises a question concerning the neural tonus
of the skeletal musculature. Since Brondgeest's experiment
neural tonus has been demonstrated to exist in various muscles
and to be of reflex origin. It has however remained a question
whether all skeletal muscles habitually exhibit a reflex tonus
or only some of them. Various experiments (Heidenhain,
Wundt) failed to discover reflex tonus in the muscles examined
by them. If the reciprocal innervation of antagonistic muscles
which obtains in so many reflexes obtains also in the tonic
reflexes maintaining neural tonus in muscles, it is obvious that
when one muscle of an antagonistic pair exhibits reflex tonus
its antagonist will not exhibit reflex tonus, but on the contrary
a slight degree of reflex inhibition. We have as yet no clear

VIII] REFLEX SYSTEMS — TONIC AND PHASIC 305

evidence on this. The feeble steady excitation which is the
sign of reflex tonus is often difficult to demonstrate. Feeble
steady inhibition would be even less easy to detect But
the selective distribution of the jerk-phenomena, under the
ordinary conditions employed for their elicitation, to single
members of antagonistic couples e. g.^ glutaeus^ vasto-crureus^
massetery and their absence, under those conditions, from the
opposite members of the couples, is suggestive that, under the
condition taken, reflex tonus may be confined to one member
of each antagonistic pair, namely to that member which is then
in reflex tonic operation, e.g. counteracting gravity for the
preservation of an habitual pose of the animal.

I have laid some stress on the broad resemblance between
the movements elicitable from the motor cortex and those of
local spinal reflexes. There are broad differences as well.

Spinal reflex movements suggest fairly obviously protective,
procreative, or visceral functions on the one hand, and on the
other the main movements of the progression habitual to the
animal. They seem to refer to stimulation of noci-ceptive or
sexual skin nerves or visceral afferent fibres, as though initiated
by these. They carry little unequivocal reference to " touch."
The existence of spinal reflexes elicited by pure " touch " —
apart from that noxious touch evoking scratching-reflexes or
eye-blinking — appears to me not established in respect to the
normal spinal cord. Similarly in the cat and dog after decere-
bration no purely auditory stimulus in my experience excites
a reflex,* nor do visual, though the optic tracts and their mid-
brain connections have been spared in the decerebration. On
the other hand various movements elicitable from the motor
cortex carry the significance of possible responses to tactual,
auditory, or visual stimuli ; for instance, the closure of the hand,
the pricking of the ear, the opening of the eyes, and turning of
the head in the direction of the gaze.

Combination of cortical reaction with spinal reflex seems
patent in certain reactions of the dog. Thus, the normal dog

* I have only seen it do so when the decerebrate animal has been under large
doses olatropin.

3o6 REACTIONS OF THE MOTOR CORTEX [Lect.

can be seen to, as it were, release, direct, and cut short a
scratching reflex (v. s. p. 289). Darwin*^ draws attention to
a phase of canine behavior in regard to defaecation. ** Dogs
after voiding their excrement often make with all four feet a
few scratches backward, even on a bare stone pavement, as if
for the purpose of covering up their excrement with earth, in
nearly the same manner as do cats.'' In the spinal dog defae-
cation is similarly followed by a number of vigorous backward
kicks with the hind limbs. The fore limbs I have not been able
to observe because the spinal transection has not lain far enough
headward to liberate those limbs for free reflex action. But this
movement in the hind limb follows as a reflex in the spinal dog
practically invariably in immediate sequence to reflex evacua-
tion of the faeces. In the normal dog it is, as Darwin remarked,
not invariable ; and it is often not in immediate sequence to the
evacuation. The reflex evidently shows modification by cerebral
direction and control.

Finally, it seems to me that the number of reflex actions
which are " neutral " to each other, in the sense expressed in
Lecture II, is less with the cerebral cortex present than without
it. This amounts to expressing concretely an inference that the
cerebral cortex augments the motor solidarity of the creature.
Since there is more solidarity as well as more diversity in those
movements of an animal which are directed to its outer environ-
ment than to its inner — meaning by this latter the fraction of
environment embraced within its own pulmono-digestive cavity
— the representation of visceral movement in the cortex will
be relatively slight and chiefly concern parts where alimentary
canal opens on outer surface.

The reactions of receptor-organs which respond to stimuli
from a distance tend especially to have large cortical representa-
tion. These receptors tend more than others to control the
skeletal musculature of the creature as a whole. The contribu-
tion made by the cerebral hemispheres to the solidarity of the
motor creature is largely traceable to their bringing to bear on
other reflexes the unifying influence of the reactions of the
•* distance-receptorsy This statement may in its baldness appear

VIII] GUIDANCE OF SPINAL REFLEXES 307

doctrinaire; of that character I hope to relieve it somewhat in
the next following Lecture.

As to the meaning of this whole class of movements elicitable
from the so-called "motor" cortex, whether they represent a
step toward psychical integration or on the other hand express
the motor result of psychical integration, or are participant in
both, is a question of the highest interest, but one which does
not seem as yet to admit of satisfactory answer. In regard to
the relatively restricted problem in view in these lectures,
namely, the simpler elements of the nervous integration of
animal reaction, the motor reactions elicitable from the so-
called " motor " cortex furnish evidence confirmatory of points
mentioned before in regard to lower reflex action. This is in-
teresting, since they must be admitted to be movements of higher
order than any of those others. Nevertheless they arc to my
thinking merely fractional movements ; movements which rep-
resent but parts of the nervous discharge which emanates from
the brain under the normal working of its unmutilated whole.
The results before you must appear a meagre contribution
toward the greater problems of the working of the brain ; their
very poverty may help to emphasize the necessity for resorting
to new methods of experimental inquiry in order to advance
in this field. New methods of promise seem to me those lately
followed by Franz, Thomdyke, Yerkes, and others ; for instance,
the influence of experimental lesions of the cortex on skilled
actions recently and individually, i. e. experientially, acquired.
Despite a protest ably voiced by v. Uexkiill, comparative
psychology seems not only a possible experimental science
but an existent one. By combining methods of comparative
psychology {e. g. the labyrinth test) with the methods of
experimental physiology, investigation may be expected ere
long to furnish new data of importance toward the knowledge
of movement as an outcome of the working of the brain.

joa THE DOMDUXCE OF THE BRAIN [Lbct.

LECTURE IX

THE PHYSCXjOCHCAL FOSHIO^ AND DOMINANCE
OF THE BRAIN

Tbe pcmxtive LCflUrSC Toe <TinlHr :
die gnpf-cniiiwi ucipgus sjsGeoi ^ tne <
afthefaKtcr. Ngnrom inrryjtim of the
cjqulve tirfaffi. UTrhnr^ of the fcHeiiMjquIie fidcL
menti of the ret epfijf - dgms of the "leaKfing*
ifffiiirtf lecepCDB of tfae trjafiin^ irj^iiir iito s
'^Distance-recsptaB:'' die pra|kaeiice of
iiitrTf i nnn al paths Ijrfcjn|^i»g to *" efistmce-feegptots^" ''Distance-
l e tj e pims " initiate ^ e mii e ut irjrti o ni > ConMim i iiate iy leactiops;
straas adective tooe of liie senntiDns ad|onct to fhfin- Receptite
nmge and JDComoti o a . The * head ^ as ph ys i n l ogirally co o ceS f e<L
PiupniMjepCi^e arcs nritrd sernm'Luily to other aics. Close fon^
tnnal < imiiritinn between the centripetal impulses from mosdes and
from the iabfitnth* Tonic iHIr iw (of postme* etc.} and compen-
satuii irilriw are chjiJj'ter&Kii reactioiB of tins combined Sfstem.
Nervous [nce g ratio Q of tiie fwgmmtal series. Restrktioa of s^
mnitif dEstnbatKXL a £Ktar in mt^iation. The ceiebeDom is the
main jgjinj^lKWk of the pfopuo-ceptive sjstcnk Tlie cendjmm is the
gangfion of the * disiaiirr > i e ce pt D fs>"

We may now attempt to gather from the various notions, how-
ever tragmentar\-, that have occupied us, some general con-
ceptioa of the neural architecture of an animal as a whole;
though of course only in its motor aspect, for its truly sensorial
aspects we have hardly had before us. The problem is too
dinicult for me to expect much success. Yet it will repay us if
from the attempt we glean something at least of one cardinal
feature of the scheme, namely, the dominance attained by one
limited set of neural segments, the brain, over all the rest.

We must allow ourselves at certain points some repetition of
considerations already urged, in order to draw from them now
in new juxtaposition some further significance.

The primitiTe reflez-arc. — If we seek for a reflex-arc of sim-
plest construction it is true we find in some unicellular organisms,

IX] THE PRIMITIVE REFLEX-ARC 309

e.g. Vorticella, a mechanism which resembles a nervous arc and
is quite simple. This mechanism, composed from a single cell,
shows differentiation into three parts respectively, — receptive^
conductive^ and effective. In Vorticella the receptive element is
the ciliated peristome; a stimulus reaching these cilia at the
free end of the cell excites contraction of the myoid filament at
the fixed end of the celL Similarly with the individual cells of
Poteriodendron (Verworn). In multicellular organisms of low
organization like mechanisms occur. In Actinia there are
ectoderm cells which have externally a receptive hairlet and
internally a contractile fibre, and this latter contracts when the
receptive hairlet is stimulated.

In view of such cases it might have seemed likely that in more
highly developed organisms examples would have been forth-
coming in which the differentiation of the parts of a single cell
would have advanced further still and produced something yet
more akin to a simple reflex-arc such as is considered typical of
the true nervous system itself. That expectation is not realized,
What we find as the simplest arc in the organisms which possess
a true nervous system is that the conductor mediating between
receptor and effector is itself a separate cell intercalated between
a receptive cell and an effector cell. At each end this separate
conductive cell breaks up into branches. The branching at the
receptive end places it in communication not with one but with
several receptor cells. This must allow stimuli at a number of
receptive points to combine by summation to a conjoint effect
By this means the threshold of reaction will be lowered and the
organism in that respect become more sensitively reactive to
the environment. At the deep, 1. e. effector, end the branching
of the conductive stem places it in touch not with one effective
cell but with many. Thus, again, there must result lowering of
the threshold — of what we may term the effective threshold.
The contraction of a single muscle-fibre in a muscle is practi-
cally ineffective where the resistance and mass of the muscle and
its load are great as compared with the power of a single muscle-
fibre. But by its branching the motor neurone obtains hold
of many muscle-fibres. This must tend to lower the effective

310 THE DOMINANCE OF THE BRAIN [Lect.

threshold of reaction, and thus again the organism is rendered
more delicately responsive to stimulation by its environment.

But — and it is a striking fact — we do not know of any reflex-
arc in which in fact the nervous conductor connecting receptor to
effector is formed from end to end of ope single neurone. The
length of the conductor seems always to include at least two
neurones in succession. A moment's reflection reminds us that
such arrangements as Vorticella, Poteriodendron, and the neuro-
muscular cells of Hydra and Actinia do not exhibit the germ of
a feature that we have already considered fundamental in the
construction of the reflex nervous system. The cases cited do
not exhibit even in germ the co-ordinative mechanism which is
attained by the principle of the common path. Such cases con-
fine each effector to the use of one receptor only, and confine
each receptor to the use of one effector only. But we saw that
a great principle in the plan of the nervous system is that an
effector shall be at the behest of many receptors, and that one
receptor shall be able to employ many effectors. We saw further
in respect to this that there are two conditions which the ner-
vous system satisfies. One is that the effector is at the behest
of various receptors which can use it simultaneously and use it
harmoniously all in more or less the same way. Thus an advan-
tage accrues in that their reactions sum, even though the re-
ceptors may be of different modality ; and by summation the
threshold is lowered and the organism more sensitized to the
environment. This arrangement cannot be obtained by the uni-
cellular mechanisms instanced above. It can only be obtained
by the formation of a common path, and the formation of a
common path can only be rendered possible by having a con-
ductor of pluricellular length. And there is another condition
which the nervous system satisfies The unicellular reflex-arc
— if reflex-arc it can be called — not only admits no opportu-
nity for pluricellular summation but also none for the second
function of the jointed reflex-arc of pluricellular length, namely
'* interference'' In animals of complex organization the activity
of one effector organ may interfere with the function of another.
e, g,, in the case of muscles which when contracting pull in

IX] THE PLURIRECEPTIVE SUMMATION 311

opposite directions at the same lever. We have seen how this
wasteful confusion is avoided by one receptor having power not
only to throw a particular effector into action but also to throw
the opposed effector out of action. We saw that this action it
exercises not peripherally but within the nervous system, at the
entrance to a common pafli. The unicellular reflex-arc allows
no common path. It lacks, therefore, the mechanism which
renders possible the two great co-ordinative processes oi plufu
receptive summation and of interference. Without these the
nervous system is shorn of its chief powers to integrate a set
of organs or an organism.

It is therefore a significant thing that in the nervous system
there is not only no instance of the reflex triune — receptor,
conductor, and effector — being formed of one cell only, but
also no indubitable instance where the middle link, the con-
ductor, is even itself formed of one cell (one neurone) only. In
other words, we know of no instance in the nervous system of a
reflex-arc so constructed as not to include a junction between
one neurone and another neurone. And the rule is apparently
always that at such junctions not only does one neurone meet
another, but several neurones converge upon another and make
of the latter a common path.

The dilfiise nerrona systam, the gray-centred nervous system,
the central nervcns system a part of the latter. The term " nerve-
centre" is sometimes abused, yet seems in several ways apt
A keynote regarding that part of the nervous system which is
termed "the central" seems that it is wholly pieced together
into one system. The nervous system in its simplest forms is
diffuse — a number of scattered mechanisms performing merely
local operations with much autonomy save that they have com-
munication with their immediate neighbors across near bound-
aries. The co-ordination effected by the diffuse nervous system
is not adapted to compass the quickly combined action of dis-
tant parts. It is slow, and it throws en route the effectors of
intermediate regions into action. It is ill suited, therefore, to
produce the integration of a large and complex individual as
a whole, or even to integrate large differentiated portions of an

312 THE DOMINANCE OF THE BRAIN [Lect.

individual. Yet the co-ordination it brings about in its own
local field may be strikingly effective. A co-adjustment though
simple and restricted may be not less perfect than one involving
wide and complex neural mechanism. The co-ordination of a
peristaltic movement of the bowel is, as shown by Bayliss and
Starling, even when managed exclusively by the local diffuse
nervous system, capable of the perfect taxis of two muscular
coats arranged antagonistically in the viscus. It directs a relaxa-
tion of the one co-ordinately with a contraction of the other ; it
exhibits a primitive but none the less perfect form of " recipro-
cal innervation."

This diffuse system seems the only one in such an organism
as Medusa. But in higher animals a system of longer direct
connections is developed. And this latter is " synaptic," that is,
possesses the adjustable junctions which belong characteristically
to "gray matter." This synaptic system co-existing with the
diffuse in various places dominates the latter. Thus it controls
and overseers the actions of the local nervous system of the vis-
cera, and heart, and blood-vessels, which even in the highest
animal forms remain diffuse.

The synaptic nervous system has developed as its distinctive
feature a central organ, a so-called central nervous system ; it is
through this that it brings into rapport one with another widely
distant organs of the body, including the various portions of the
diffuse nervous system itself.

That portion of the synaptic system which is termed '* cen-
tral " is the portion where the nervous paths from the various
peripheral organs meet and establish paths in common, /. e.
^' common paths'' It is therefore in accord with expectation that
we find the organ in which this meeting occurs situated fairly
midway among them all, i. e. centrally. In bilaterally symmetri-
cal animals this organ would be expected to lie where it does,
namely, equidistant from the two lateral surfaces of the animal,
and to exhibit as it does, laterally symmetrical halves united by
a number of nervous cross ties bridging the median line. This
central nervous organ contains almost all the junctions existent
between the multitudinous conducting arcs. In it the afferent

IX] THE DIFFUSE NERVOUS SYSTEM 313

paths from receptor-organs become connected with the efferent
paths of effector-organs, not only those adjacent to their own
receptors but, through '* intemuncial'* Q. Hunter, 1778) paths,
with efferent paths to effector-organs remote. This central
" exchange " organ is therefore well called the central nervous
s>'stem. In the higher Invertebrata it is known as the longitu-
dinal nerve-cord with ganglia, supraoesophageal, suboesopha-
geal, etc. ; in Vertebrata it is known as the spinal cord and brain.
Under these different anatomical names the same physiological
organ is designated. It would be more convenient for the biol-
ogist were one general term for it in use. We have seen that
it is not merely a meeting place where afferent paths conjoin
with efferent, but is, in virtue of its physiological properties,
an organ of reflex reinforcements and interferences, and of
refractory phases, and shifts of connective pattern; that it
is, in short, an organ of c<hordination in which from a con-
course of multitudinous excitations there result orderly acts,
reactions adapted to the needs of the organism, and that these
reactions occur in arrangements (patterns) marked by absence
of confusion, and proceed in sequences likewise free from
confusion.

By the development of these powers the synaptic system
with its central organ is adapted to more speedy, wide, and
delicate co-ordinations than the diffuse nervous system allows.
Out of this potentiality for organizing complex integrration there
is evolved in the synaptic nervous system a functional g^rading
of its reflex arcs and centres. Thus, with allied reflexes, the
mechanism of the common path knits together by plurireceptive
summation not only the separate individual stimuli of similar
kind, e.g. tangoreceptive or photoreceptive received from some
agent as this latter becomes prepotent in the environment ; but
it knits together separate stimuli of even wholly different recep-
tive species. C. J. Herrick has shown that in Ameiurus nebuUh
sus (cat-fish) the reaction of the animal to stimulation of the
barblets by meat is a reaction to a twofold stimulus, a chemical
and a mechanical, and he finds that these two reactions mutually
reinforce. Nagel reports a similar case with the tentacle of the

314 THE DOMINANCE OF THE BRAIN [Lect.

Actinian, Aiptasis saxicola. v. Uexkiill finds that the Gift-
zangen of Echinus acutus react only when a chemical and a
mechanical stimulus are combined. The several qualitatively
different properties of an object which is acting as stimulus are
thus combined and reinforce each other in eliciting appropriate
reaction. By this summation reflex complication in Herbart's
sense is made possible. A touchstone for rank of a centre in
this neural hierarchy is the deg^ree to which paths from separate
loci and of different receptive modality are confluent thither.
Indicative of high rank is such functional position as relieves
from " local work " and involves general responsibility, e. g, for
a series of segments or for the whole body. The " three levels,"
of Hughlings Jackson is an expressive figure of this grading of
rank in nerve-centres.

Integrative action of the nervona system in the segment and
in the segmental series. In animal organisms of any consider-
able complexity a division of the body into segments, metameres,
is widely found. By the occurrence of separating constrictions
or sepiments, or through the regular repetition of appendicular
structures, subdivisions of the body are established which sev-
erally possess analogues of functions possessed more or less
similarly by the other subdivisions but also severally possess
functional unity. Such is this functional unity and completeness
that in some instances a metamere comes to be independent of
the total organism, and able to lead a separate existence. The
nervous system it is which largely gives functional solidarity to
the composite collection of unit lives and organs composing the
individual metamere. Further, the linkage of the several meta-
meres into one functional whole is largely of nervous nature.
The integrative function of the nervous system is seen to perfec-
tion in the welding together of metameres into the unity of an
animal individual. The kind of nervous system employed for
this is the synaptic system. Although the nerve-net system is
retained even in the highest vertebrates, it is then confined to
unsegmentally arranged musculature, e,g. visceral and vascular.
In the skeletal musculature, where segmental arrangement holds,
the nervous system is synaptic. It is not surprising therefore

IX] THE CENTRALIZED NERVOUS SYSTEM 315

that in metameric animals the nervous system, especially its
synaptic part, should strikingly exhibit that metamerism.

Various schemes of metamerism have been evolved. Where
it is radiate so that each segment bears exactly similar relations
to the common axis and to the other segments, the opportunity
for dominance of one segment over the rest is slight. The con-
ditions of life for each segment are practically those which are
the average for all. The mouth, for instance, lies equidistant from
them all. Evolution toward higher differentiation of the whole
metameric individual and toward more intricate welding of its
parts into one, is at a disadvantage in these radiate forms as
compared with its opportunity in the great g^roups of Arthropoda
and Vertebrata where the metameres are ranged serially along a
single axis, the longitudinal axis of the organism. With fore
and aft arrangement of its segments the animal body has its first
opportunity for really high differentiation. Certain of the seg-
ments of necessity lie nearer to the mouth than do others;
moreover certain segments come to habitually lead, that is to
say go foremost, during the animal's active locomotion.

In the integrating function of the nervous system a segmental
arrangement of its functions is frequently apparent. It makes
itself felt in tAvo ways. Firstly, the various separate and differ-
ent elements of the segment are knit together by nervous ties.
Secondly, where kindred functions are exercised in successive
segments, so that throughout a series of segments one set of
organs forms a more or less functionally homogeneous system,
these organs are combined by interrelated nervous arcs. But
particular systems of organs common to all or many metameres
of an individual present special differentiation of their (unction
in particular metameres. In this manner the organism is built
up of component segments possessing resemblance one to an-
other, but presenting also specializations peculiar to certain seg-
ments. Hence the segmental arrangement forms a convenient
basis not merely for anatomical but for physiological description.
And in dealing with the special problems of integration by the
nervous system, especially those of the synaptic nervous system,
analysis can employ two co-ordinate sets of descriptive factors

5rf THE DOMKAXCE OF THE BRAIN [Lect.

— cne :3K m^mwic , t!ie oc3icr &e Eae of organs of analogous
Sii a ca co m ifr^qj aikn^ t^ series of segments. In the two
great amtnai frescos jccst aneabocied the latter ordinate is longi-
imfmaiTy rnrMfrr . w*arTe tiK isftdxridnal segment is extended
traasverseiy. T!Mr SBiLysss dms proceeds formally somewhat in
die same jow ms- tiK aaahrss of a plane ^^are by rectangular

Tke amtml n er v o us system, though
'cfanisntv is ytX one single hannoni-
: cotpfex whole. To analyze its action we
tarn to the receptOF-ocgaKS» far to diem is traceable the initia-
tk» of the rea c t ioaE S of &e centres. These organs £dl naturally
into t hr ee OBtain groups^ (fistribated in three main fields, each
wou being mzpcrentiy cucnoKstanced.

Muhicdliilar amnials rega r ded broadly throughoat a vast
range of animal types are cdhilar masses presenting to die en-
virottment a snrCace sheet of cdb, and under that a cellular
balk more or less screened firom the environment by the surface
sheet. Many of die a gen ci e s by which the environment acts on
the organism do not penetrate to the deep cells inside. Bedded
in the snr&ce sheet are numbers of ncefior cells constituted in
adaptation to die stimuli ddivered by environmental agencies.
The mmdertyin^ tissues devoid of these receptors are not devoid
of all receptor-organs ; they have other kinds apparently specific
to them. Some agencies act not only at the surface of the
organism but penetratively through its mass. Of these there
are for some apparently no receptors adapted, for instance, none
for the Rbntgen rays. For others of more usual occurrence re-
ceptors are adapted. The most important of these deep ade-
quate agents seems to be mass acting in the mode of weight and
mechanical inertia invoK-ing mechanical stresses and mechanical
strain. Moreover, the organism, like the world surrounding it,
is a field of ceaseless change, where internal energy is continu-
ally being liberated, whence chemical, thermal, mechanical, and
electrical effects appear. It is a microcosm in which forces are
at work as in the macrocosm around. In its depths lie receptor-
organs adapted consonantly with the changes going on in the

IX] METAMERISM AND NERVOUS SYSTEM 317

microcosm itself, particularly in its muscles and their accessory
apparatus (tendons, joints, walls of blood-vessels, and the like).

There exist, therefore, two primary distributions of the recep-
tor-organs, and each constitutes a field in certain respects funda-
mentally different from the other. The deep field we have called
i}ci<tpropruh-ceptivefiAAy because its stimuli are, properly speaking,
events in the microcosm itself, and because that circumstance
has important bearing upon the service of its receptors to the
organism.

Rlchnaiiii of the eztero-o«ptiTe field in reoepton ; oompamthre
poverty of the intero-ceptive. The surface receptive field is
again subdivisible. It presents two divisions. Of these one
lies freely open to the numberless vicissitudes and agencies of
the environment That is to say, it is co-extensive with the so-
called external surface of the animal. This subdivision may be
termed the extero-ceptive field.

But the animal has another surface, its so-called internal^
usually alimentary in function. This, though in contact with
the environment, lies however less freely open to it It is
partly screened by the organism itself. For purposes of retain-
ing food, digesting and absorbing it, an arrangement of common
occurrence in animal forms is that a part of the free surface is
deeply recessed. In this recess a fraction of the environment
is more or less surrounded by the organism itself. Into that
sequestered nook the organism by appropriate reactions gathers
morsels of environmental material whence by chemical action
and by absorption it draws nutriment. This surface of the
animal may be termed the inter(hceptive. At its ingress several
species of receptors are met with whose "adequate" stimuli
are chemical {e.g. taste organs). Lining this digestive chamber,
this kitchen, the intero-ceptive surface is adaptpd to chemical
agencies to a deg^ree such as it exhibits nowhere else. Compara-
tively litde is yet known of the receptor-organs of this surface,
though we may suppose that they exhibit refined adaptations.
But the body-surface in this recess, though possessed of certain
receptors specific to it, is sparsely endowed as contrasted with
that remainder of the surface (the extenhceptive surface) lying

3i8 THE DOMINANCE OF THE BRAIN [Lect.

open fully to the influences of the great outer environment
The afferent nerve-fibres in the sympathetic system as judged by
their number in the white rami are comparatively few ; Warring-
ton's ** recent observations show this conclusively. The poverty
of afferent paths from the intero-ceptive field is broadly indicated
by the fact that we know no wholly afferent nerve-trunk in the
sympathetic (Langley's " autonomic ") system, though such are
common enough in the nervous system subserving the extero-
ceptive arcs ; and that in the latter system we know no wholly
efferent nerve-trunk, whereas in the sympathetic such exist, e.g.
the cervical sympathetic trunk.

The extero-ceptive field far exceeds the intero-ceptive in its
wealth of receptor-organs. This seems inevitable, for it is the
extero-ceptive surface, facing outward on the general environ-
ment, that feels and has felt for countless ages the full stream of the
varied agencies forever pouring upon it from the outside world.
Mere enumeration of the different species of receptor-organs
recognizable in it suffices to illustrate the importance of this
great field. It contains specific receptors adapted to mechanical
contact, cold and warmth, light, sound, and agencies inflicting
injury {noxa). Almost all these species of receptors are distrib-
uted to the extero-ceptive field exclusively ; they are not known
to exist in the intero-ceptive or in the proprio-ceptive fields.

It is an instructive exercise to try to classify the stimuli ade-
quate for the receptors of the extero-ceptive field. Each animal
has experience only of those qualities of the environment which
as stimuli excite its receptors; it analyzes its environment in
terms of them exclusively. Doubtless certain stimuli causing
reactions in other animals are imperceptible to man ; and in a
large number of cases his reactions are different from theirs.
Hence it is impossible for man to conceive the world in terms
more than partially equivalent to those of other animals. Hu-
manly, the classification of adequate stimuli can be made with
various departments of natural knowledge as its basis. Physics
and chemistry can be taken as basis usefully in a number of
cases where the sources of stimulation are known to those more
exact branches of experimental science. But in several ways a

IX] CLASSIFICATION OF RECEPTORS 319

physico-chemical scheme of classification of stimuli lacks sig-
nificance for physiology. Thus, in the case of the noci-ceptive
organs of the skin, those receptors — probably naked nerve-
endings — are non-selective \n the meaning that they are excit-
able by physical and chemical stimuli of diverse kind, radiant,
mechanical, acid, alkaline, electrical, and so on, so that a classi-
fication according to mode of exciting energy on the one hand
fails to differentiate them from each of a number of more
specialized other groups (tango-receptors, chemo-receptors, etc.)
from which biologically they are quite different, and on the
other hand that classification apportions them, though physio-
logically a single group, to a whole series of different classes.
A physiological classification deals with them more satisfac-
torily. Physiological criteria can be applied which at once
separate them from other receptors and yet show their affinity
one to another. Thus, physiologically the stimulus which ex-
cites these end-organs must, whatever its physical or chemical
nature, possess, in order to stimulate them, the quality of tending
to do immediate harm to the skin. Further, the reflex they
excite (i) is prepotent; (ii) tends to protect the threatened part
by escape or defence ; (iii) is imperative; and (iv) if we include
psychical evidence and judge by analogy from introspection, is
accompanied hy pain.

Here what we may call the physiological scheme of classifi-
cation proves the more useful at present And similarly it proves
useful with a group of stimuli that may be termed " distance-
stimuli," to which we must turn presently. The key to the
physiological classification lies in the reaction which is produced.
But the physico-chemical basis of classification also has its uses,
and especially with those manifold receptors of the extero-
ceptive field which possess highly developed accessory struc-
tures that render them selectively receptive — and among these
are some of the most highly adapted and important receptors,
e,g, photo-receptors, possessed by the organism. It is to the
extero-ceptive field that these belong.

Nerrous integration of the segment. The edifice of the whole
central nervous system is reared upon two neurones, — the aflfer-

520 THE DOBUNANCE OF THE. BRAIBr \ rwnr

exit root-cell and the efferent root-ceiL Tliesc: fiunx t4ii^ piHar^
a{ a fundamental reflex arcii. And on the jundiaiL h^»^■"■^
these two are superposed and fiinctinnaily set, mecfistsijr or im^
mediately, ail the other nenrai arcs, even those of the ^i^ ' t - ^ of
the cerebrum itsei£ The private receptor p^hs 2aat the com<-
mon effector paths are in the Chordsita. ^dhered up ixr 2. gn*^ *^
oerve-tnmk for each segment: Close to the central ueiwuus
organ, however, there o ccur s in the segmental nerves of man^
Vertebrates a cleavage of the private receptor pitffas fiom Ae
common effector paths. A dorsal spinal nerve of centripetal
conduction and a ventral of centrifiigal conductxan resoibL
Among the afferent root-cells (affis^oit spinal root of Verte^
brates) in each segment are quota from the cetBra-^aptkae
(cutaneous) and from the propruM:epttDg (deep) fidk^ la
many segments diere is a third quotum^ mteffh^aftsaey^ firaoE the
visceral field* This visceral con s t ituen t of the ^^inal ggagfioa is
not present in all segments «md is probably, eves in tbi»e seg-
ments in wfaidi it is present, numerically the weaken of the
diree components^ Cranial, caudal, smd other segments exists
therefore, m. which die total afferent nerve of die segimeiit is
extero*t:eptive and proprio-ceptive but not mtenxepttve. In
Ae remaining segments it is intenxeptrve aa well as extero-
ceptive and proprio-ceptive, and in diese its function is therefore
fundamentally threefold.

The efferent segmental nerve conversely radiates outward
from the central end of the afferent nerve and the central nervous
organ to the various effector organs at the surface and in the
depth of the segment. Function does not, however, strictly
re'^f>ect the ancestral boundaries of segments. Among the
efferent fibres in the ventral root are a number that extend
quite beyond the boundaries of the segment in which the spinal
root is placed. These pass to the viscera and muscles of the
skin. They embouch not directly into their effector organs, e.g.
intestinal muscle-wall, pilomotor muscle, etc., but into ganglia
of the sympathetic system. In these ganglia, although not gray
matter in the same sense as spinal cord and brain, axone-endings,
perikarya, and dendrites are nevertheless found. By its distri-

IXJ NERVOUS INTEGRATION OF SEGMENT 321

butioo to the cells in such a ganglion and by being distributed
in many cases to more than one such ganglion, a single con-
stituent efferent path in the ventral spinal root obtains access
to a very large number of effector organs. These ganglia seem,
therefore, mechanisms for the distribution of nerve-impulses.
We have seen (p. 310) how by such widening of distribution the
threshold of effective reaction is lowered. But though adapted
for distribution of nerve-impulses there is no evidence that these
ganglia can serve for the regulation of them in the same sense as
does the gray matter of the spinal cord with its synapses of
variable resistance and connection. Prominent among the inte-
grating connections intrinsic to each segment itself are conduct-
ing paths from the extero-ceptive field to the *' final common
paths" for the skeletal musculature. Thus, in the mammal we
laid it down (Lect. V, p. 157) as a general rule that " for each
afferent root there exists in immediate proximity to its own
place of entrance into the cord, i. e. in its own segment, a reflex
motor path from skin to muscle of as low resistance as any open
to it anywhere."

The extero-ceptive arcs appear in most segments less closely
connected with the visceral musculature than with the skeletal
musculature. The intero-ceptive arcs appear in most segments
less closely connected with the skeletal musculature than with
the visceral. In physiological parlance a resistance to conduc-
tion seems intercalated between the two. But both extero-
ceptive and intero-ceptive fields easily influence through their
nervous arcs the musculature of the blood vascular Organs. So
also do the receptors of the proprio-ceptive field itself; and these
latter are in particularly close touch with the skeletal muscula-
ture, exerting tonic influence on it. In certain segments these
general relations are modified in special ways. Thus, in those
segments where the intero-ceptive and extero-ceptive fields con-
join, e.g. at the mouth and the cloaca, closer nervous connec-
tions exist between the intero-ceptive arcs and the skeletal
musculature, and conversely between the extero-ceptive arcs and
the visceral musculature. Thus stimuli acting on the pharyngeal
receptors evoke or inhibit activity of skeletal muscles subserving

122 THE DOMINANCE OF THE BRAIN [Lkx

respiration and de^ntition ; stimoli to the doocai miK'mj evoke
movements of the caudal skeletal moscks; and 90 fbrth.

It is not merely specific difference between the recep t ots of
the extero-ceptive field and those of the intero ccptii e wbidi
brings the former into closer relationship widi the siceietal
musculature. Receptors of the one and the same ^ i rr i rs , if
they lie in the extero-ceptive field, work skeletal nwismlialme:
if they lie in the intero-ceptive, work voceral mnscolatizrc.
Thus the chemo-receptors on the outer sar£K:e of the head
(gustatory of the barblets of fish) excite reflexes vdiich move
the body round, bringing the mouth to the morsel ; while the
similar chemo-receptors within the mouth excite reflex swaDow-
ing without outward movement of the animal (C J. Henick).

Receptors of the same specific system, where they He close
together, mutually reinforce reaction. On die contrary, where
members of two different systems lie close together, r.^. tango-
receptor and noci-ceptor, in one and the same piece of sldn, they,
as mentioned above, often have conflicting mntnal relation.
One relationship between receptor arcs of the same species
may be particularly noted. Receptors symmetrically placed on
opposite sides of the segment, especially if distant firom the
median plane, excite reactions which mutually '^ conflict." Thus,
when a noci-ceptor \s stimulated on the right side of the tail of
the spinal dog or cat or lizard, the reaction moves the organ to
the left The symmetrical receptor on the left side does the
converse. The two reactions thus conflict. And the like holds
true for the many right and left symmetrical receptors which
initiate exactly converse reactions.

But a group of special cases is formed by reactions inidated
from receptors distributed at or near to the median line. Stimu-
lation of such a small group of receptors at the median line in
many cases evokes a bilateral movement which is s>Tnmetri-
cal, e. g. a touch on the decerebrate frog's lip in the median
line causes both fore limbs to sweep forward synchronously
over the spot. The median overlap of the distribution of the
afferent fibres of the dorsal spinal roots may be connected with
this.

IX] REFINEMENTS OF RECEPTORS 323

Special refinements of the receptors of the ''leading " segments.
As the receptors that are excitable by the various adequate
agencies, e.g, mechanical impact, noxa, radiant energy, chemical
solutions, etc., are traced along the series of segments, it is found
that in one region of the longitudinal segmental series remark-
able developments exist

In motile animals constituted of segments ranged along a
single axis, e.g. Vertebrata, when locomotion of the animal goes
on, it proceeds for the most part along a line continuous with
the long axis of the animal itself, and more frequently in one
direction of that line than in the other. The animal's loco-
motor appendages and their musculature are favorably adapted
for locomotion in that habitual direction. In the animal's pro-
gression certain of its segments therefore leacL The receptors
of these leading segments predominate in the motor taxis of the
animal. They are specially developed. Thus, in the earth-
worm, while all parts of the external surface are responsive to
light, the directive influence of light is g^reatest at the anterior
end of the animal. The leading segments are exposed to exter-
nal influences more than are the rest Not only do they receive
more stimuli, meet more " objects " demanding pursuit or avoid-
ance, but it is they which usually yfrr/ encounter the agents
beneficial or hurtful of the environment as related to the indi-
vidual. Pre-eminent advantage accrues if the receptors of these
leading segments react sensitively and differentially to the agen-
cies of the environment And it is in these leading segments
that remarkable developments of the receptors, especially those
of the extero-ceptive field, arise. Some of them are specialized
in such degree as almost obscures their fundamental affinity to
others distributed in other segments. Thus, among the system
of receptors for which radiation is the adequate agent, there
are developed in one of the leading segments a certain gfroup,
the retinal, particularly and solely, and extraordinarily highly,
amenable to radiations of a certain limited range of wave-length,
These are yi^^ photo-receptors, for which light and only light, e.g.
not heat, is the adequate stimulus. In like manner a certain
group belonging to the system receptive of mechanical impacts

324 THE DOMINANCE OF THE BRAIN LLect-

attains such susceptibility for these as to react to the vibrations
of water and air that constitute physical sounds. The retina is
thus a group of glorified " warm-spots," the cochlea a group of
glorified " touch-spots." Again, a group belonging to the sys-
tem adapted to chemical stimuli reach in one of the leading
segments such a pitch of delicacy that particles in quantity un-
weighable by the chemist, emanating from substances called
odorous, excite reaction from them.

The refined receptors of the leading segments are "distance-
receptors." The after-coming segments form a motor train actuated
chiefly by the " distance-receptors." It is in the leading segments
that we find the " distance-receptors'' For so may be called the
receptors which react to objects at a distance. These are the
same receptors which, acting as sense-organs, initiate sensations
having the psychical quality termed projicience. The receptor-
organs adapted to odors, light, and sound, though stimulated
by the external matter in direct contact with them, — as the
vibrating ether, the vibrating water or air, or odorous particles,
— yet generate reactions which show "adaptation," e, g, in
direction of movements, etc., to the environmental objects at a
distance, the sources of those changes impinging on and acting
as stimuli at the organism's surface. We know that in ourselves
sensations initiated through these receptors are forthwith " pro-
jected " into the world outside the ** material me." The proji-
cience refers them, without elaboration by any reasoned mental
process, to directions and distances in the environment fairly
accurately corresponding with the ** real " directions and dis-
tances of their actual sources. None of the sensations initiated
in the proprio-ceptive or intero-ceptive fields possess this prop-
erty of projicience. And with the distance-receptors considered
simply as originators of reflex actions, their reflexes are found
to be appropriate to the stimuli as regards the direction and
distance of the sources of these latter. Thus, the patch of
light constituting a retinal image excites a reflex movement
which turns the eyeball toward the source of the image and
adjusts ocular accommodation to the distance of that source
from the animal itself. Even a negative stimulus suffices. The

IX] DISTANCE RECEPTORS 325

shadow of the hand put out to seize the tortoise excites, as it
blots the retinal illumination, withdrawal of the animal's head to
within the shelter of the shell.

How this result of" distance" has been acquired is hard to say.
The net effect is reached in various ways, and with very various
gain in the degree of •' distance " acquired. By long vibrissae
certain tango-receptors obtain excitation from objects still at a
distance from the general surface of the organism. By reduc-
tion of their threshold value of stimulus, certain other receptors
akin to tactual, inasmuch as their adequate stimuli are mechan-
ical, become responsive to vibratory movements of water and air
so as to react to physical sounds whose sources lie remote from
the animal. Certain chemo-receptors acquire so low a threshold
that they react not merely to food and other substances in con-
tact with them in mass, but react to almost inconceivably diluted
traces of such, traces which drift off from the objects and per-
meate the environment through long distances, as so-called
odors, before impinging upon the delicate receptors in ques-
tion. The leading segments thus come to possess not only
taste, but taste at a distance, namely smell. In such cases it
seems chiefly by lowering of their threshold that these receptors
of the leading segments have been brought to react to objects
still remote from the organism.

The " distance-receptors " seem to have peculiar importance
for the construction and evolution of the nervous system. In
the higher grades of the animal scale one part of the nervous sys-
tem has, as Gaskell insists, evolved with singular constancy a
dominant importance to the individual. That is the part which
is called the brain. The brain is always the part of the nervous
system which is constructed upon and evolved upon the ** distame-
receptor'' organs. Their effector reactions and sensations are
evidently of paramount importance in the functioning of the
nervous system and of the individual. This seems explicable,
at least partly, in the following manner.

An animal organism is not a machine which merely trans-
forms a quantum of energy given it in potential form at the out-
set of its career. It has to replenish its potential energy by

326 THE DOMINANCE OF THE BRAIN [Lect.

continued acquisition of suitable energy-containing material from
the environment, and this material it has to incorporate in itself.
Moreover, since death cuts short the career of the individual
organism, the species has to be maintained, and for that in most
higher organisms there is required accession of material (ga-
metic) from another organism (of like species) to rejuvenesce
a portion of the adult, which portion then cast off leads a new
individual existence. To satisfy, therefore, the primary vital re-
quirements of an animal species, actual material contact with
certain objects is necessary; thus, for feeding, and in many
cases for sexual reproduction.

In these processes of feeding and conjugation the non-
distance-receptors play an important and essential part. But
ability on the part of an organism to react to an object when
still distant from it allows an interval for preparatory reactive
steps which can go far to influence the success of attempt either
to obtain actual contact or to avoid actual contact with the
object. Thus, we may take in illustration the two sets of selec-
tive chemo-receptors, the gustatory and the olfactory. Both are
responsive to certain chemical stimuli which reach them through
solution in the moist mucous'membranes of the mouth and nose.
No odorous substance appears to be tasteless, and if the thresh-
old value for olfaction and for taste be measured respectively,
the threshold for the former as determined in weight of dissolved
material is lower than for the latter. The former is the distance-
receptor. Animal behavior shows clearly that in regard to
these two groups of receptors the one subserves differentiation
of reaction, /. e, swallowing or rejection, of material already
found and acquired, e*g. within the mouth. The other, the
distance-receptor, smell, initiates and subserves far-reaching
complex reactions of the animal anticipatory to swallowing,
namely, all that train of reaction which may be comprehensively
termed the quest for food. The latter foreruns and leads up to
the former. This precurrent relation of the reaction of the
distance-receptor to the non-distance receptor is typical.

The "distance-receptors " initiate anticipatory, i, e, precurrent,
reactions. I ventured above to use the word " attempt." Just as

IX] DISTANCE RECEPTORS 327

a salient character of most of the reactions of the non-projicient
receptors taken as sense-organs is " affective tone^" i, e. physical
pain or physical pleasure, so *^ conative feeling" is salient as a
psychical character of the reactions which the projicient or
distance-receptors, taken as sense-organs, guide. As initiators
of reflex movements the action of these latter is characterized
by tendency to work or control the musculature of the animal
as a whoU^ — as a single machine, — to impel locomotion or to
cut it short by the assumption of some total posture, some atti-
tude which involves steady posture not of one limb or one
appendage alone, but of all, so as to maintain an attitude of the
body as a whole. Take, for instance, the flight of a moth toward
a candle, the dash of a pike toward a minnow, and the tense
steadiness of a frog about to seize an insect. These reactions
are all of them excited by distance-receptors. Though in the
one case the musculature is impelled to locomotion toward the
stimulus (positive phototropism), in the other restrained (in-
hibited) from locomotion. Whether the reaction be move-
ment toward or movement away from (positive or negative) or
whether it be motion or its restraint (excito-motor or inhibito-
motor) does not matter here. The point here is that in both re-
actions the skeletal musculature is treated practically as a whole
and in a manner suitably anticipatory of a later event That is
far less the case with the non-projicient receptors. The decere-
brate frog changes the whole direction of its path of locomotion
when a visual obstacle is set in its way, but a skin impact ex-
cites a movement in a small field of musculature only, e. g. the
eyelid blinks on corneal contact, the foot flexes at a digital noxa;
where the part itself cannot well move itself musculature acces-
sory to it but distant from it is moved. Thus the hind limb is
swept over the flank on irritation there, or the fore limb over
the snout on irritation there. But in these cases the movement
induced is merely local and does not affect the body as a whole.
Sufficient intensity (we may include summation under intensity)
of a stimulus can of course impel the whole creature to move-
ment even through a non-projicient receptor. A decerebrate frog
touched lightly between the scapulae will lower its head at first

328 THE DOMINANCE OF THE BRAIN [Lect.

touch, and again more so at a second ; at a third will, besides
lowering the head, draw the front half of its trunk sli^dy
backward; at a fourth the same movement with stronger re-
traction; at a fifth give an ineffectual sweep with its hind or
fore foot ; at a sixth a stronger sweep ; at a seventh a feeble
jump; at an eighth a free jump, and so forth. Considerable
intensity or summation is required to evoke a reflex reaction
of the skeletal musculature as a whole from these cutaneous
receptors. The projicient receptors and their reflexes once
gone, even intense stimuli do not readily move or arrest the
creature as a whole. It is relatively difficult to get the '' spinal "
frog to spring or swim. Co-ordinate movement of the crea-
ture as a whole is then obtained by general stimulation {i. e,
plurireceptive summation), or if by localized stimulation the
stimulus must be intense. Thus the spinal frog will swim
when placed in water at 36^ C. The warm water, forms a noci-
ceptive stimulus to the receptors of the immersed body*surface
generally.

Bxtensive intemnnoial paths of " distanoe-receptors.'* Con-
formably with the power of the " distance-receptors " to induce
movements or postures of the individual as a whole we find the
neural arcs from these receptors particularly wide and far-reach-
ing. The nerve-fibre that starts from the receptor does not in
many of these cases itself extend to, or send processes to, the
mouths of the ''final commofi paths," Instead of doing so it
ends often far short of them, and forms connection with other
nerve-fibres (internuncial paths), which in their turn reach dis-
tant " final common paths." This arrangement involves an inter-
calation of gray matter between the " private receptor" path and
the ** final common path " not only at the mouth of the latter, but
also where the internuncial path itself commences. The signifi-
cance of this seems that the internuncial path is itself a " common
path, and therefore a mechanism, of accommodation" Its com-
munity of function is not so extensive as that of a " final com-
mon path," not co-extensive for instance with all the receptors
of the body, as would appear the case with a motor-nerve to a
skeletal muscle. Yet it furnishes a path for use by certain sets

IX] PRECURRENT REACTIONS 329

of receptors in common. In Mustelus the nerve paths from the
retinal and from the olfactory receptors converge toward the roof-
nucleus of the mid-brain, whence passes the long mesencephalon
spinal path to the spinal motor nuclei. The inference is that
conjoint stimulation of eye and nose exert a combined influence
and impinge together on the spinal motor machinery. Similarly
the Reissner fibre *^ may serve as an intcmuncial path between
paths coming in from olfactory and visual receptors on the one
hand and the spinal motor common paths from the spinal cord
to the muscles on the other. Another instance of an internun-
cial path is the so-called '* pyramidal tract " characteristic of the
mammalian nervous system. It furnishes a path of internuncial
character common to certain arcs that have arisen indirectly
from various receptors of various species and are knitted to-
gether in the cerebral hemisphere. Another instance is the
path from the thalamus to the post-central convolution (Mott,
Tschermak, and others).

Preourrent reactionB. Consamnuitory reaotionB. It might
seem at first that all motor reflexes may be grouped into those
that tend to prolong the stimulus and those that tend to cut
it short Consideration shows that such a grouping expresses
the truth but partially. We arg^ued above that the " distance-
receptors " induce anticipatory or precurrent reactions, that is,
precurrent to final or consummatory reactions. The reflexes of
certain non-projicient receptors stand in very close relation to
" consummatory " events. Thus the tango-receptors of the lips
and mouth initiate reflex movements that immediately precede
the act which for the individual creature viewed as a conative
and a sentient agent is the final consummatory one in respect to
nutriment as a stimulus, namely, swallowing. Similarly with the
grustato-receptors and their reactions. The sequence of action
initiated by these non-projicient receptors is a short one : their
reflex leads immediately to another which is consummatory.
Those receptors of the chelae of Astacus, Homarus, etc., which
initiate the carrying of objects to the mouth, or again the tango-
receptors of the hand of the monkey when it plucks fruit and
carries it to the lips, give reactions a step further from the con-

330 THE DOMINANCE OF THE BRAIN [Lect.

summatory than those just instanced These reactions are all
steps toward final adjustments^ and are not themselves end-points.
The series of actions of which the distance-receptors initiate the
earlier steps form series much longer than those initiated by the
non-projicient Their stages, moreover, continue to be guided
by the projicient organs for a longer period between initiation
and consummation. Thus in a positive phototropic reaction the
eye continues to be the starting place of the excitation, and
in many cases guides change in the direction not only of the
eyeball but of the whole animal in locomotion as the reflex
proceeds. The mere length of their series of steps and the vicis-
situdes of relation between bodies in motion reacting on one
another at a distance conspire to give to these precurrent re-
flexes a multiformity and complexity unparalleled by the reflexes
from the non-projicient receptors. The reaction started by
" distance-receptors " where positive not only leads up to the
consummatory reactions of the non-projicient, but on the way
thither associates with it stimulation of other projicient recep-
tors, as when, for instance, a phototropic reaction on the part of
a Selachian brings the olfactory organs into range of an odorous
prey, or, conversely, when the beagle sees the hare after run-
ning it by scent In such a case the visual and olfactory recep-
tor arcs would be related as " allied " arcs (Lecture IV), and
reinforce each other in regard to the mesencephalo-spinal path,
or in higher mammals the " pyramidal " or other pallio-spinal
path. It is easy to see what copious opportunity for adjust-
ment and of side connection such a reaction demands, consist-
ing as it does of a number of events in serial chain, each link a
modification of its predecessor.

Strong affective tone an accompaniment of consammatory re-
actions. We may venture to turn briefly to the psychical aspect
of such sequences. To consummatory reactions affective tone
seems adjunct much more than to the anticipatory, especially the
remotely anticipatory of the projicient sense-organs. Thus the
affective tone of ** tastes " is strong. The reaction initiated by a
noci'Ceptor (pp. 226-23 1 ) is to be regarded as consummatory. The
application of an irritant to the flank of a frog evokes a movement

IX] AFFECTIVE TONE 331

of the leg adapted to at once remove that stimulus from the skin
of the flank. Or again, an irritant applied to the skin of the
foot evokes a movement of the foot away from that stimulus.
In both cases the reaction is a consummatory one» because it is
calculated of itself to be final. To judge by our own introspec-
tion the affective tone adjunct to these reactions is strong. They
instance strong affective tone pertaining to consummatory re-
actions. The affective tone of the reactions of the projicient
receptors is less marked : physical pleasure or pain can hardly
be said to accompany them. Not of course that they are wholly
unrelated to affective tone. The relative haste with which an
animal when hungry approaches food offered to the visual field
suggests that conation attaches to the visual reaction by asso-
ciation through memory with affective tone. By associative
memory a tinge of the affective tone of the consummatory reac-
tion may suffuse the anticipatory. The latter becomes indirectly
a pleasure-pain reaction. The neutral tango-receptive reactions
of the feet of the tortoise hastening stumblingly towards its food
may in this way be imbued with a tinge of affective tone derived
from the affective tint of the leading reflex, namely the visual,
which itself has thus memorial association with a consummatory
reflex of strong affective tone. Examples of this type of reaction
furnished by new-born animals are given by Lloyd Morgan.^**
When " after a few days the new-bom chick leaves ladybirds
unmolested while he seizes wasp-larvae with increased energy "
he affords evidence that reactions of his projicient receptors
have acquired a new value, and that value is made up mediately
of affective tone. How they have acquired it or what exact
nature their new attribute has is not our question. It is enough
here that in regard to certain stimuli the new value — the mean-
ing — which the projicient sensation has obtained has reinforced
greatly the conative intensity of the reaction to the stimulus. It
has given the stimulus increased force as a spring of precurrent
actions aimed at a final consummatory one. It has given this not
by altering the external stimulus, nor the receptor-organ, but by,
among other alterations, altering internal connections of the re-
ceptor arc. Thus it is that, be it by associative memory or other

332 THE DOMINANCE OF THE BRAIN [Lect.

processes, the reactions of the " distance-receptors " come in
higher animals to reveal a conative driving force which is per-
haps the end for which these psychoses exist.

Nor are the series of reactions, short though they be, which
the non-projicient receptors initiate wholly devoid of conative
appearance. They show adaptation as executive of steps toward
an end. Food, sexual consummation, suitable posture, preser-
vation from injury, are ends to which their direction leads, as
with the longer series of actions due to projicient receptors
reacting to objects at a wider horizon. It is rather that the lat-
ter afford a freer field for the winning more subtle adjustments
with wider application of associative memory. In the latter
there is more scope for the play of mind, — mind it may be of
such elementary grade as to be difRcult for us to picture in its
operations.

We may suppose that in the time run through by a course
of action focussed upon a final consummatory event, oppor-
tunity is given for instinct, with its germ of memory however
rudimentary and its germ of anticipation however slight, to
evolve under selection that mental extension of the present
backward into the past and forward into the future which in the
highest animals forms the prerogative of more developed mind.
Nothing, it would seem, could better ensure the course of action
taken in that interval being the right one than memory and
anticipatory forecast: and nothing, it would seem, could tend to
select more potently the individuals taking the right course than
the success which crowns that course, since the consummatory
acts led up to are such — e,g. the seizure of prey, escape from
enemies, attainment of sexual conjugation, etc. — as involve the
very existence of the individual and the species. The problem
before the lowlier organism is in some slight measure shadowed
to us by the difficulties of adjustment of reaction shown by the
human child. The child, although his reactions are perfect
within a certain sphere of his surroundings, shows himself at the
confines of that sphere a little blunderer in a world of over-
whelming meaning. Hence indeed half the pathos and humour
derivable from childhood.

IX] RECEPTIVE RANGE 333

It is the long serial reactions of the "distance-receptors"
that allow most scope for the selection of those brute organisms
that are fittest for survival in respect to elements of mind. The
" distance-receptors " hence contribute most to the uprearing of the
cerebrum. Swallowing was above termed a consummatory reac-
tion. Once through the maw, the morsel is, we know by
introspection, under normal circumstances lost for conscious-
ness. But it nevertheless continues to excite receptors and
their nervous arcs. The significant point is that the object has
passed into such a relation with the surface of the organism
that *' conation " is no longer of advantage. The naive notion
that when we have eaten and drunken we have fed is justified
practically. No effort can help us to incorporate the food
further. Conation has then done its all and has no further
utility in respect to that food taken. It is significant that all
direct psychical accompaniment of the reactions ceases abruptly
at this very point The immediately precedent reactions that
were psychically suffused with strong affective colour pass ab-
ruptly over into reactions not merely affectively neutral but void
— normally — of psychical existence altogether. The concomi-
tance between certain nervous reactions and psychosis seems an
alliance that strengthens the restless striving of the individual
animal which is the passport of its species to continuance of
existence.

Receptive range. The ascendency of "distance-receptors " in
the organization of neural function may be partly traceable to the
rA2\Avc frequency of their use. Although it would be incorrect
to assess the value of an organ by the mere frequency with which
it is of service, yet caeteris paribus that seems a fair criterion.
The frequency with which a receptor meets its stimuli is, other
things being equal, proportionate to the size of the slice of the
external world which lies within its " receptive range** Although
in a fish, for instance, the skin with its tango-receptors is much
larger in area than arc the retinae with their photo-receptors, the
restricted " receptive-range " — the adequate stimulus requiring
actual proximity — of the former gives a bx smaller slice of the
stimulus-containing world to the skin than pertains to the eyes.

334 THE DOMINANCE OF THE BRAIN [Lect.

In the case of the eye not only is the slice of environment pertain-
ing to it at even a short distance more wide and high than that of
the skin, but it is at each moment multiplied by the third dimen-
sion. There arise in it, therefore (caeteris paribus)^ in unit of time
many more stimulations, with the result that the receptor-organ
of " distant " species receives many more fresh stimuli per unit of
time than does the receptor-organ of restricted receptive range.
The greater richness of the neural construction of the photo-
receptive system than of \he tango-recepdve accords with this.
Thus in the photo- receptive system the so-called " optic nerve "
(which since it is the second neural link and therefore to some
extent a " common path," presents numerical reduction from the
first or private path in the retina itself) contains more conduc-
tive channels (nerve-fibres) in man (1,000,000, Krause) than are
contained in the whole series of afferent spinal roots of one side
of the body put together (634,000, Ingbert^ **), and of these
latter the cutaneous afferent fibres form only a part, and of that
part the tango-receptive fibres themselves form only a fraction.
The large number of the channels in the retinal path is no doubt
primarily indicative of spatial differentiations of the receptive sur-
face, but that spatial diffefentiation is itself indicative of the
numbers of the stimuli frequenting that receptive field.

Locomotion and " receptive range/' Locomotive progression
and distance receptivity are two phenomena so fundamentally
correlated that the physiology of neither can be comprehended
without recognition of the correlation of the two. Evidence
is forthcoming from ontogeny and phylogeny. The elaborate-
ness of the photo-receptive organs of the flying Insecta corre-
sponds with the great power of these forms to traverse space.
When the Brachiopod passes from a motile wandering life to a
fixed sedentary one its " eyes " degenerate and go. The free-
swimming Ascidia with fin-like motor organs and semi-rigid axial
notochord, affording elasticity and leverage, bears at its anterior
end a well-formed photo-receptor organ (eye^ and a well-formed
otocyst (head proprio-ceptor). Connected with the nerves of
these, the anterior end of its truly vertebrate central nervous
system has a relatively large " brain." Thence extends back-

IX] RECEFnVE RANGE AND LOCOMOTION 335

ward aibng the body a spinal cord. Suddenly its free-swimming
habit is exchanged for a sedentary ; by adhesive projections from
its head, it attaches itself permanently to some fixed object At
once there ensues a re-adaptive metamorphosis. Degeneration
sets in concurrently in its locomotive musculature, its eye, its
otocyst, its brain, and its cord. These vanish as by magic save
that a fraction of the brain remains as a small ganglion near the
mouth. The sessile creature retains, so far as can be judged
from their microscopic structure, only some gustatory (?) recep-
tors round the mouth, and some tango- receptors (? noci-ceptors)
in the tegument, connected doubtless with an irregular diffuse
subtegumental layer of unstriped muscle-tissue. Experimental
observations seem wanting on the point, but we may presume
that in this metamorphosis the receptive range of Ascidia dwindles
from dimensions measurable by all the distance through which
its free motile individual floats and swims, to a mere film of the
external world, say a millimeter deep, at its own surface, espe-
cially round its mouth, and unextended by succession of time,
save passively by the mere flowing of the water. Such instances
illustrate the fundamental connection between the function of the
skeletal musculature and that of the " distance-receptors." Did
we know better the sensual aspects of these cases the more sig-
nificant doubtless would be the comparison.

The "head" as phyaiologioally oonceived. As regards the
objects acting on the organism at any moment through its
receptors, the extension of environmental space — the animal's
receptive range — is not equal in all directions as measured from
the organism itself. The extension is greater in the direction
about the "leading" pole. Thus, the reactions initiated at the
eye forerun reactions {cf, Loeb's Ketten-reflexe) that will in due
time come to pass through other receptor-organs. The visual
receptors are usually near the leading pole, and so placed that
they see into the field whither progression goes. And simi-
larly with the olfactory receptors. The motor train behind, the
elongated motor machinery of the rest of the body, is there-
fore from this point of view a motor appendage at the behest of
the distance-receptor organs in front. The segments lying at

336 THE DOMINANCE OF THE BRAIN [Lect.

the leading pole of the animal, armed as they are with the
great ''distance" sense-organs, constitute what is termed the
" head."

The proprlo-oeptiTe •jmtnm and the head. We may now at-
tempt to enquire whether this dominance of the leading segments
which is traceable in the receptors of the extero-ceptive field ap-
plies in the field of reception which we termed the proprio-ceptive.
We arrived earlier at the notion that the field of reception which
extends through the depth of each segment is differentiated
from the surface field by two main characters. One of these
was that while many agents which act on the body surface are
excluded from the deep field as stimuli, an agency which does
act there is mass, with all its mechanical consequences, such as
weight, mechanical inertia, etc., giving rise to pressures, strains,
etc., and that the receptors of this deep field are adapted for
these as stimuli. The other character of the stimulations in this
field we held to be that the stimuli are given in much greater
measure than in the surface field of reception, by actions of the
organism itself, especially by mass movements of its parts.
Since these movements are themselves for the most part reac-
tions to stimuli received by the animal's free surface from the
environment, the proprio-ceptive reactions themselves are results
in large degree habitually secondary to surface stimuli. The
immediate stimulus for the reflex started at the deep receptor
is thus supplied by some part of the organism itself as agent.

In many forms of animals, e,^, in Vertebrates, there lies in
one of the leading segments a receptor-organ (the labyrinth)
derived from the exteroceptive field, but later recessed off from
it ; and this is combined in action with receptors of the proprio-
ceptive field of the remaining segments. This receptive organ,
like those of the proprio-ceptive field, is adapted to mechanical
stimuli. It consists of two parts, both endowed with low recep-
tive threshold and with refined selective differentiation. One
part, the otolith organ, is adapted to react to changes in the
incidence and degree of pressure exerted on its nerve-endings
by a little weight of higher specific gravity than the fluid other-
wise filling the organ. The other part, the semicircular canals.

IX] THE PROPRIOCEPTIVE SYSTEM 337

reacts to minute mass movements of fluid contained within it
These two parts constitute the labyrinth. The incidence and
degree of pressure of the otoliths upon their receptive bed
change with changes in the posiHan of the segment in which
the labyrinth lies, relatively to the horizon line. Movements of
the segment likewise stimulate the labyrinthine receptors through
the inertia of the labyrinthine fluid and the otoliths. By the
labyrinth are excited reflexes which adjust the segment (and
with it the head is usually immovably conjoined) to the horizon
line. And other parts are similarly reflexly adjusted by it
Thus, the refined photo-receptive patches in the head — the
retinae — which conduct reflexes delicately differential in re-
gard to space, appropriate for stimuli higher or lower or to right
or to left in the photo-receptive patch, depend in their conduct
of these upon a more or less constant standardization of their
own normals of direction in regard to the horizon line. These
photo-receptive patches are set movably in the head; by the
action of muscles they can retain their bearing to the hori-
zon, although the head itself shifts its relation to the horizon.
The control of these muscles lies largely with the labyrinth. The
labyrinth produces a compensatory eyeball reflex. Thus in the
head segments the labyrinth effects reflex movements analogous
to that which the proprio-receptive nerves from the extensor
muscles of the knee excites in the leg segments, reflexes restor-
ing an habitual posture that has been departed from.

And from the above it seems clear that there is another fea-
ture of resemblance between the labyrinthine receptor and the
proprio-ceptors of the limb. Stimulation of the labyrinth must
in preponderant measure be given not by external agents directly
but by the reaction of the organism itself Posture and move-
ment of the head are the immediate causes which stimulate the
labyrinth, whether or not they be part of a total movement or
posture of the whole individual. Such movement is most fre-
quently an active one on the part of the animal itself. Thus,
when Ascidia becomes sedentary and its locomotor muscalature
atrophies its otocyst disappears. But an animal's active move-
ment is in its turn usually traceable as a reaction to an cnviron-

338 THE DOMINANCE OF THE BRAIN [Lect.

mental stimulus affecting the receptors at the surface of the
animal. Thus the labyrinthine receptors like the proprio-ceptors
in other segments, are stimulated by the animal itself as agent,
though secondarily to stimulation of the animal itself via some
cxtero-ceptor.

And there is another point of likeness between lab3ninth re-
flexes and those of the proprio-ceptors of the limb and other
segments. The proprio-ceptors of the limbs appear productive
of certain continuous, that is tonic^ reflexes. Thus, in the decere-
brate dog the tonic extensor rigidity of the leg appears reflexly
maintained by afferent neurones reaching the cord from the
deep structures of the leg itself. Similarly, if the knee-jerk be
accepted as evidence in the spinal animal of a spinal tonus in
the extensor muscle, this tonus seems maintained by afferent
fibres from the extensor muscle itself, since the knee-jerk is
extinguished by severence of those fibres. Again, the rapidity
of onset of rigor mortis in a muscle is speedier when its tonus
prior to death has been high. Section of the afferent roots ot
the limb prior to death delays onset of rigor mortis *^ in that
limb as judged by stiffness at the knee ; but that delay is not
observable when skin-nerves only have been severed. The
labyrinthine receptors appear likewise to be the source of cer-
tain maintained, that is tonic, reflexes. Destruction of the laby-
rinth also delays the onset of rigor-mortis in the muscles to
which its field of tonus can be traced. Ewald has shown that
each labyrinth maintains tonus especially in the neck and trunk
muscles and in the extensor-abductor limb-muscles of the
homonymous side.

In regard to these tonic reflexes it is difficult to see how a
steady mechanical stimulus can continue to elicit a reflex con-
stantly for long periods. If we take sensation as a guide, a touch
excited by constant mechanical pressure of slight intensity fades
quickly below the threshold of sensation. It is said that a spinal
frog may even be crushed by mechanical pressure without ex-
citing from it a reflex movement provided that the pressure
be applied by very slowly progressive increments. The office
of a receptor would seem to be, placed across the line of a

IX] TONIC REFLEXES 339

stream of energy, to react under the transference of energy
across it, as for instance from the environment to the organism,
or vice versa. We have many instances in which the Uving
material adapts itself to, and maintains its own equilibrium
under, different grades of environmental stress, treating each
fairly continuous or slowly altering grade as a normal zero.
The slow changes of barometric pressure on the body surface
originate no skin-sensation, thpugh they are much above the
threshold value for touch. There streams constantly from the
body through the skin a current of thermal energy much above
the threshold value of stimuli for warmth sensations ; yet this
current evokes under ordinary circumstances no sensation. It
is the stationary condition, the fact that the transference of
energy continues at constant speed, which makes it unperceived.
The receptor apparatus is not stimulated unless there is a change
of rate in the transference, and that change of rate must occur
in most cases with considerable quickness, otherwise there is a
mere unperceived shift in the stationary equilibrium which forms
the resting zero of the sensual apparatus. Over and over in the
elicitation of reflexes as well as in the artificial excitation of
nerve or muscle we meet this same feature. Both for sensa-
tion and for reflex action a function in the threshold value of
stimulus is time as well as intensity and quantity. If a weak
agent is to stimulate, its application must be abrupt But in
the tonic reflexes whose source lies at the proprio-ceptors and
the labyrinth a weak stimulus, although apparently unchanging,
seems to continue to be an effective stimulus.

The proprio-ceptors and the labyrinthine receptors seem to
have in common this, that they both originate and maintain tonic
reflexes in the skeletal muscles. And they, at least in some
instances, reinforce one another in this action. Thus the tonus
of the extensor muscle of the knee in the cat and dog appears
to have a combined source in the proprio-ceptors of that muscle
itself and in the receptors of the homonymous labyrinth. The
tonus of skeletal muscles is an obscure problem. Its mode of
production, its distribution in the musculature, its purposive sig-
nificance, are all debateable. The steadiness and slight inten-

340 THE DOMINANCE OF THE BRAIN [Lect.

sity of the contraction constituting the tonus render its detection
difficult Part of the discrepancy between the experimental
findings may be traced to the supposition that a reflex tonus if
present is present in all muscles at all times. A single muscle
examined for reflex tonus has been taken to represent all
muscles under all conditions, although the answer has been
sometimes positive and sometimes negative.

It appears to me likely that reflex tonus is the expression of
a neural discharge concerned with the maintenance of attitude.
In many reflex reactions the eflect is movement and the muscles
are dealt with as organs of motion. In these cases the stimuli
and the reactions both of them are short-lived events. But
much of the reflex reaction expressed by the skeletal muscu-
late is postural. The bony and other levers of the body are
maintained in certain attitudes both in regard to the horizon, to
the vertical, and to one another. The frog as it rests squatting
in its tank has an attitude far diflierent from that which g^vita-
tion would give it were its musculature not in action. Evi-
dently the greater part of the skeletal musculature is all the time
steadily active, antagonizing gravity in maintaining the head
raised, the trunk semi-erect, and the hind legs tautly flexed.
Innervation and co-ordination are as fully demanded for the
maintenance of a posture as for the execution of a movement
This steady co-ordinate innervation antagonizes gravitation and
other forces, e.g. as in currents of water. In these tonic as in
other reflexes antagonistic muscles co-operate co-ordinately.
There is nothing to show that reciprocal innervation does not
obtain in the one class of reflex as in the other. If so, it be-
comes easily intelligible that the slight reflex contraction termed
skeletal tonus should under given conditions be found in some
muscles and not in others. The slight reflex contraction will be
accompanied by reflex inhibition of the antagonistic muscles.
For reflex tonus to be the expression of a neural discharge
which maintains attitude accords well with the ascription of its
source to the proprio-ceptors, including the labyrinth. Those
are exactly the receptors which, functioning as sense-organs,
initiate sensations of posture and of attitude (Bonnier). And it

IXJ SKELETAL MUSCULAR TONUS 341

accords also with the share in the production and regrulation of
skeletal tonus which the cerebellum has (Luciani's atonid) and
the cerebrum.

Naturally, the distinction between reflexes of attitude and
reflexes of movement is not in all cases sharp and abrupt Be-
tween a short lasting attitude and a slowly progressing move-
ment the difference is hardly more than one of degree. Moreover,
each posture is introduced by a movement of assumption, and
after each departure from the posture, if it is resumed, it is re-
verted to by a movement of compensation. Hence the taxis of
attitude must involve not only static reactions of tonic mainte-
nance of contraction, but innervations which execute reinforcing
movements and compensatory movements. In all this kind of
function the proprio-ceptors of the body generally and of the
labyrinthine receptors in the head appear to co-operate together
and form functionally one receptive system.

This system as a whole may be embraced within the one term
" proprio-ceptive." Our inquiry regarding it is now, whether
that part of it which is situate in the leading segments, namely
its labyrinthine part, exerts preponderance in the system as in
the extero-ceptive system do the extero-ceptors situate in the
leading segments. It must be remembered of the extero-ceptive
system that even in the segments which are not the leading
segments its receptors considered as sense organs produce
sensations that have some projicience ; and that in animals pro-
vided with outstanding skin appendages, e.g. hair, the tango-
reflexes are to a slight extent reactions to objects at a distance.
This germ of distance reaction and projicience of sensation in
the extero-ceptors of the ordinary body-segments is developed
in the extero-ceptors of the leading segments into the vast
distance-reactions of the eye and the absolute projicience of
vision. But the proprio-ceptors of the limb and body segments
exhibit no germ of distance-reaction nor of projicience of sensa-
tion. And the specialized proprio-ceptor organ of the leading
segment (the labyrinth) is similarly not a distance-receptor;
although some of its sensations seem projected into the environ-
ment as well as referred to the organism itself, to the " material

i42 THE DOMINANCE OF THE BRAIN {JUbct.

me.** Any predcmiiitartcr dus piopriaoepCor in tihe headmg

fine in virtue of llie qnalitjr of leacdon at a dril a nrr If pte"
emitieft^ impottant to llie organism as a whde tts pte-emiae^
of importance rests on odier gromids than does tfie inqKiftance
of the great distance-receplofs, — llie cri&ctoiy, the visual, and
the auditoiy.

Apostnre of die animal as a wiiole — a total posture — bas
much a complex built lip of postures of portions of the animal
— segmental postures (Bonnier) — as is the total movement of
the animal — its locomotion — compounded cf sq;mental move-
ments. With the hinder part of its spinal cord alone mtact die
frog maintmns a posture in its hind Umbs. These limbs are kq»t
flexed at hip, knee, and ankle. When displaced from diat pos-
ture they return to it But if die animal be rolled over on its
back it makes no attempt to right itsd£ The decerebrate frog
widi its labyrinths intact and dieir arcs still in connection with
the skeletal musculature maintains the well-known attitude before
mentioned* If inverted it at once reverts to diat The laby-
rinth keeps the world right-side up for the organism by keeping
the organism right-side up to its external world. The cranid
receptors control the animal's iotal posture as do receptors of
the hinder musculature the segmefital posture of the hind limbs
when but the hind end of the spinal cord remains.

Thus the labyrinthine proprio-ceptors are largely the equi-
librators of the head, and since the retinal patches are movably
attached (in mobile eyeballs) to the head, and since each retina
has its normals of direction conforming with those of the head,
these equilibrators of the head are closely connected by nervous
arcs with the musculature maintaining tHe postures of the eye-
balls. The posture of the head in many animals is dependent
on the musculature not of the head segments themselves but of
a long series of segments behind the head. In many forms the
motor organs that steadily maintain or passingly modify the
position of the head in regard to the external world — con-
veniently indexed by the line of direction of gravitation — are
contributed by the skeletal musculature of many post-cranial

IX] TOTAL AND SEGMENTAL ATTITUDE 343

segments. Hence the labyrinthine receptor is in touch with
all the segments of the body, and these in a measure may be
regarded as appended to the otic segment. Destruction of the
labyrinth in the fish, the frog, the pigeon, the dog produces not
only malposture of the eyeball and the head, but of the limbs
and body as a whole. The " knock-out blow," where the lower
jaw conveys concussion to the otocyst, reduces in a moment
a vigorous athlete to an unstrung bulk of flesh whose weight
alone determines its attitude, if indeed a reactionless mass can
be described as possessing attitude at all.

The labyrinthine receptors and their arcs give the animal its
definite attitude to the external world. The muscular receptors
give to the segment — e.g. hind limb — a definite attitude less
in reference to the external world than in reference to other
segments, e.g. the rest of the animal. Our own sensations
from the labyrinth refer to some extent, as said above, to this
environment, that is, have some projected quality ; our muscular
sensations refer to the body itself, e. g. contribute to perceptions
of the relative flexions or extensions of our limbs. The arcs of
the proprio-receptor of the leading segments control vast fields
of the skeletal musculature, and deal with it as a whole, while
the arcs of the proprio-ceptors of the other segments work with
only limited regions of the musculature. Hence, in conformity
with this the proprio-ceptor of the leading segments possesses
long internuncial paths, for instance, bulbo-spinal from Deiter's
nucleus proceeding to all levels of the spinal cord.

We traced the reactions of proprio-ceptors of the limb to
bear habitually a secondary relation to the reactions of the
extero-ceptors of the limb. Similar secondary relation is evi-
dent also between the reactions of the proprio-ceptor of the
leading segments (the labyrinth) and the reactions of the
extero-ceptors of those segments. These latter extero-ceptors
were seen to be distance-receptors, and the reactions of distance
receptors were seen to be signalized by their anticipatory char-
acter. From secondary association vnth these distance-receptors
the reactions of the labyrinth come in their turn to have antici-
patory character. They retain, however, their own special

344 THE DOMINANCE OF THE BRAIN [Lect.

features of equilibration and tonus. The locomotion of an
animal impelled by its eye toward its prey involves co-operation
of the labyrinth with the retina. And the tonic labyrinthine
reflex which maintains an attitude may be just as truly an an-
ticipatory reaction as any movement is. The steady flexed pos-
ture of the frog directed toward a fly seen on the aquarium
wall is a co-ordinate innervation securing preparedness for the
seizure of the food. Its character is as truly anticipatory as is
that of any movement. We might speak of the animal as " at
rest," but it is the tense quietude of the hunter watching quarry
rather than rest, such as supervenes in sleep and other conditions
where active innervation is actually relaxed or reflex action is
truly in abeyance.

NenroQs integration of a segmental series. By longitudinal
integration short series of adjoining segments become in respect
to some one character combined together, so as to form in
respect to that character practically a single organ. It is con-
venient to speak of such reflex reactions, conflned from start to
finish to a single integrated set of segments, as " short reflexes "
giving " local reactions." Thus the vertebrate appendages called
limbs are plurisegmental, but the individual segments constitut-
ing the limb form in respect of the limb a functional group of
such solidarity that their reactions in the limb are at any one
time unitary.

The reflexes that extend beyond the limit of such a group are
on the other hand conveniently termed " long reflexes^ And it
is in the integration of long series, or of the whole series, of seg-
ments one with another, that, apart from psychical phenomena,
the nervous system seems to reach its acme of achievement. Here
it is that we see eminently what Herbert Spencer has insisted
on, namely, that integration keeps pace with differentiation.

In the segmental series the nervous concatenation of the
segments repeats broadly the kind of association evidenced
within each segment taken singly. Broadly taken, each seg-
ment has on the one hand a piece of the extero-ceptive field,
a piece of the proprio-ceptive field, and a piece of the intero-
ceptive field, though this last is wanting in not a few segments.

IXJ NERVOUS INTEGRATION 345

On the other, it has fractions of the skeletal, of the vascular,
and of the visceral effector organs. Each segment has muscu-
lature and glands on its outer and visceral surfaces. Some
segments have also secretors discharging into body spaces.
Each of these sets of features of the segments has in the series
of segments a nervous system of some functional homogeneity.
With these plurisegmental systems as with their unisegmental
pieces in the single segment the same harmonies of interconnec-
tion are observable. Thus, the nervous arcs embouching into
the skeletal musculature start chiefly in the extero-ceptive field
in so far as concerns execution of passing movements, in the
proprio-ceptive field in so far as concerns tonic postures; and
so on, as sketched above. If the receptors of the extero-ceptive
field are regarded from the point of view of the nature of the
agency adequate for each of their species, representatives of each
species are found in almost every segment In this way the
functional properties of the extero-ceptive field form not one but
several multisegmental organs or systems of organs. In each
segment exist receptors responsive to mechanical, chemical, and
radiant agencies respectively. There is thus formed a tango-
ceptive system to which practically every segment contributes, a
thermo-ceptive system, a noci-ceptive system ; so also a musculo-
ceptive system, and probably the receptors of the intero-ceptive
surface similarly constitute a homogeneous system, prominent
among their adequate agencies being those of chemical quality.
These systems of receptive arcs present, though more or less
compound, a solidarity of action in each system that gives each
some rank as a physiological entity.

Restriction of segmental distribution a factor in integration.
The impulse to nervous integration given by regional restriction
of a peculiar species of organ to a single segment has especial
force where that organ is of especial importance. This is the
case with effector organs subserving important actions of con-
summatory (v. s. p. 326, 329) type, e. g. a sexual appendage, or
the mouth. Such organs as these are of restricted regional dis-
tribution and subserve important reactions of consummatory type.
With the mouth is associated differentiation of organs around

346 THE DOMINANCE OF THE BRAIN [Lect.

it Many postures and movements of the organism are advanta-
geous or disadvantageous to the animal's existence mainly inas-
much as they improve or disimprove the position or attitude of
the mouth in regard to objects in the external world. Much of
the long series of movements and other reactions initiated and
guided by *' distance-receptors " themselves is by-play on the
way to a consummatory reaction which requires an appropriate
placing and attitude of the mouth. That there is only one mouth
and that of limited segmental extent involves co-ordination of the
activities of many other segments with the oral Integration of
pluri-segmental activity is effected here, as in the other cases,
mainly by the s)maptic nervous system. The fact that the
mouth is usually placed near the leading segments of the ante-
rior pole is therefore a further factor in differentiating the seg-
ments at that end from the after-coming train. Thus it comes
about that in many cases the animal consists of two portions
broadly different in character but complemental the one to the
other, the head and the trunk.

It is noteworthy that the increase of susceptibility instanced
by the distance receptors is in each case restricted to a special
patch, quite limited in area. Given a synaptic nervous system,
no single item of functional arrangement more enforces integra-
tion of an individual from its segments than the restriction of a
special kind of receptor to a single area or segment in the whole
series. The motor apparatus of many segments has then to
subserve a single segment, since that segment is provided with a
receptor of a species not otherwise possessed by the individual
at all. For integrative co-ordination of that kind the synaptic
nervous system affords in the animal economy the only instru-
ment. Only by the formation of common paths can due advan-
tage be reaped from a specially refined recipient path (private
path) of locally restricted situation.

Further, the condensed setting of a group of specialized
receptors favors their simultaneous stimulation in groups to-
gether. Stimuli even of small area then cover a number of
receptive points in the receptive sheet Thus, ocular images of
various two-dimensional shape tend to be better differentiated

IX] DIFFERENTIATION AND INTEGRATION 347

by the photo-receptors the more closely the individual photo-
receptors lie together. More data are thus gained as a basis for
differential reaction.

Further, the juxtaposition of groups of specially refined recep-
tors in one set of segments, the leading or head segments, con-
duces toward their simultaneous stimulation by several agencies
emanating from one and the same environmental object. Thus,
the property of brightness and the property of odor belonging
to an object of prey may then better excite in unison a reaction
in the distant reagent, or excite more potently than would either
property alone. And movements of the reagent itself are then
more apt to intensify simultaneously the reactions of its two
kinds of receptors. The collocation of the disparate receptors
in one region will favor that which psychologists in describing
sensations term *' complication," a process which in reflex action
has a counterpart in the conjunction of reflexes excited by recep-
tors of separate species but of allied reaction. This alliance of
reaction we have seen finds expression as mutual reinforcement
in action upon a final common path. Thus a reaction is synthe-
sized which deals with the environmental object not merely as a
stimulus possessing one property but as a " thing " built up of
properties. A reflex is attained which has its psychological
analogue in a sense percept.

The carebeliiim is the head gangilion of the proprio-oeptiTe
system. If the basis taken for classification of receptors be a
physiological one with, as its criterion, the type of reaction which
the receptors induce, separate receptive systems may be traced
running throughout the whole series of segments composing
the total organism. We have seen that such separate receptive
systems may be treated as functional unities, extending through
the segmental series. In any such system there is evident a ten-
dency for its central nervous mechanisms, that is to say, the com-
ponents of the central nervous organ which specially accrue to
the system in question, to be gathered chiefly where the most
important contribution to its receptive paths enters the central
nervous system. The receptive system in question has as it
were its focus at that place. Thus receptive neurones which

348 THE DOMINANCE OF THE BRAIN [Lect.

can influence respiratory movement enter the central nervous
organ at various segments, but the chief respiratory centre lies
in the bulb where the receptive neurones from the lung itself
make entrance and central connection, the vagal receptors being
preponderantly regulative in that function. And we have seen
that a proprio-ceptive organ (the labyrinth) in the head seg-
ments seems preponderantly regulative in those functions which
the proprio-ceptive system subserves. The central neural mech-
anism belonging to the proprio-ceptive system is preponder-
antly built up over the central connections of this proprio-ceptive
organ (the labyrinth) belonging to the head. Thither converge
internuncial paths stretching to this mechanism from the central
endings of various proprio-ceptive neurones situate in all the
segments of the body. There aflerent contributions from the
receptors of joints, muscles, ligaments, tendons, viscera, etc.,
combine with those from the muscular organs of the head and
with those of the labyrinthine receptors themselves. A central
nervous organ of high complexity results. Its size from animal
species to animal species strikingly accords with the range and
complexity of the habitual movements of the species ; in other
words, with the range and complexity of the habitual taxis of the
skeletal musculature. This central organ is the cerebellum.

The symptoms produced by its destruction or injury in whole
or in part in many ways resemble, therefore, the disturbances pro-
duced by injury of the labyrinth itself. It also influences tonus
very much as do the simple proprio-ceptive arcs themselves.
It is closely connected structurally and functionally with the so-
called motor region of the cerebral hemisphere, just as the
simpler proprio-ceptive arcs and reflexes are closely associated
with the mechanisms of extero-ceptive reactions. Knowledge is
not ripe as yet for an adequate definition of the function of the
cerebellum. Many authorities have defined it as the centre for
the maintenance of the mechanical equilibrium of the body.
Others regard it as the organ for co-ordination of volitional
movement. Spencer suggested that it was the organ of co-ordi-
nation of bodily action in regard to space, the cerebrum he
suggested being the organ of co-ordination of bodily action in

IX] THE CEREBRUM 349

respect of time. Lewandowski considers it the central organ for
the " muscular sense." Luciani, the universally acknowledged
authority on the physiology of the cerebellum, describes it as
the organ which by unconscious processes exerts a continual
reinforcing action on the activity of all other nerve-centres.

It is instructive to note how all these separate pronounce-
ments harmonize with the supposition that the organ is the chief
co-ordinative centre or rather group of centres of the reflex
system of proprio-ception. The cerebellum may indeed be
described as the head-ganglion of the proprio-ceptive system,
and the head ganglion here, as in other systems, is the main
ganglion.

The oerebmm is the ganglion of the ** distanoe-reoepton." By
the " distance-receptors " are initiated and g^uided long series of
reactions of the animal as a whole. Other receptive reactions
integrate individual segments; the reactions of the distance-
receptors integrate the whole series of segments. It is in the
sphere of reactions of these " distance-receptors " that the most
subtle and complex adjustments of the animal therefore arise.
In their neural machinery not only short arcs but long arcs, in-
volving extensive internuncial tracts, figure largely. Chains of
reaction conducive to a final reaction relatively remote are more
evident with them than with other arcs. If appeal to psychical
evidence be ventured on it is to the field of operation of the
arcs of these distance-receptors that higher feats of associative
memory accrue, and, though the phrase is hardly permissible
here except with curtailed scope, conation becomes more intel-
ligent. Finally, in harmony with the last inference, it is over
these "distance-receptors" and in connection with their reflexes
and arcs that the cerebrum itself is found. The cerebrum
constitutes, so to say, the ganglion of the " distance-receptors."
LangendorfT^^ has pointed out that a blinded frog resembles in
its reactions a firog with the cerebrum removed: the elasmo-
branch without its olfactory lobes behaves as if it had lost its fore
brain. Edinger traces the genesis of the cerebral cortex to a
distance-receptor, namely the olfactory organ.

The integration of the animal associated with these ** distance-

3SO THE DOMINANCE OF THE BRADf [Lbch

feceplDffS " €if llie leading aqyucali am bebfki^wilk paitial
jiislice ezpicsaed by tapag tfiat tibe rest cttbc ammal, so far as
its motior m a chtnq y goes, is bat flic senraatcif them, Wemig^
tm^lUie tfie form of die individiial and die diapoaiticm of die
sense-organs as primitivcfyveiysiiiqde; far instance, a spheroid
witb a digestive cavi^ and sense-organs distiibatBd especially
over tlie external surface. Soch an im^rinaiy foim «e dioald
expect under evolution to become modified. Ifassotiieocgan-
ism, its contractile mrrhamsms would obtain mrrfianirri ad-
vantage Qewngt) hy its eioi^prtion in certun (firecticms. Tbe
lei^;tliwise extennoo of die vertdxate bodjr and of its lateral
motor a^>pendj^;es, g. g. limbs, are in so far such as noi^ht lie
argued a /TMrC Under evolution in motile aiiimah adifitations
securing appropriate leverage far the oootracdie s^iparatas
appear, and length along certain aiKS is alwaqrs a <
in tbem. In animab widi sq[ments rallied along a sii^^ ;
the animal for the greater part of its kngdi comes to be one
great motor organ, complex and able to execute movements in
variom ways* but still a unitjr. The pole at which die great
** distance-receptors" (visual, olfactoiy, auditoiy) lie fa that
which, in die habitual locomotion of die animal under the action
of die motor train attached, '"leads." The animal therefore
moves habitually into that part of environmental space which
has been already explored by the distance-receptors of its own
leading segments.

The head is in many ways the individual's greater part It
is the more so the higher the individual stands in the animal
scale. It has the mouth, it takes in the food, including water
and air, it has the main receptive organs providing data for the
rapid and accurate adjustment of the animal to time and space.
To it the trunk, an elongated motor organ with a share of the
digestive surface, and the skin, is appended as an apparatus for
locomotion and nutrition. The latter must of necessity lie at
the command of the great receptor-organs of the head. The
co-ordination of the activities of the trunk with the requirements
of the head is a cardinal function of the synaptic nervous system.
Conducting arcs must pass from the cephsdic receptors to the

IXJ THE NERVOUS SUPERSTRUCTURE 351

contractile masses of the body as a whole. The spinal cord
contains these strands of conductors in vertebrates and is from
this point of view a mere appendage of the brain. A salient
feature of these conducting arcs is that the nerve-fibres from the
cephalic receptors do not run, as might perhaps a priori have
been thought natural, direct from their cephalic segment back-
wards to reach the common effector paths upon which they
embouch. Instead of having that arrangement, these fibres,
starting in the cephalic receptors, end in the gray matter of the
central nervous axis not far from their own segment Thence
the conducting arc is continued backward by another strand of
fibres, and these reach (perhaps directly) the mouths of the
final common paths in the gray matter of segments of the spinal
cord. This is the arrangement exemplified by the pulmono-
phrenic and other respiratory arcs, the depresso-splanchnic
arcs, the olfacto-phrenic respiratory arcs, the arcs between the
otic labyrinth and the muscles maintaining posture in the trunk,
and practically that of the retino-motor arcs connecting the
retina with the muscles of the neck. It gives at least one synap-
sis more than the first alternative would do. And each synapse
is an apparatus for ohordination; it introduces a '' common path''
And it is in the exercise of the distance-receptors with their
extensive range overlapping that of other receptors that the
reflexes which relate to " objects " in the sense that they are
reflexes synthesized from receptors of separate species become
chiefly established. The ramifications of the central neurones
attached to these receptors are so extensive and the reactions
they excite are so far spreading in the organism that their asso-
ciation with the reactions and central mechanisms of other recep-
tors is especially frequent and wide.

The distance-receptors are the gfreat inaug^rators of reaction.
The reduced initiation of action which ensues on ablation of the
cerebrum seems explicable by that reason. The curtailment
which ensues is indicative of damage which their removal inflicts
on reactions generated by the distance-receptor organs. By a
high spinal transection the splendid motor machinery of the
vertebrate is practically as a whole and at one stroke severed

352 THE DOMINANCE OF THE BRAIN [Lect.

from all the universe except its own microcosm and an environ-
mental film some millimeters thick immediately next its body.
The deeper depression of reaction into which the higher animal
as contrasted with the lower sinks when made spinal signifies
that in the higher types more than in the lower the great dis-
tance-receptors actuate the motor organ and impel the actions
of the individual. The deeper depression shows that as the
individual ascends the scale of being the more reactive does it
become as an individual to the circumambient universe outside
itself. It is significant that spinal shock hardly at all affects the
nervous reactions of the intero-ceptors (visceral system) ; and
that it does not affect the intero-ceptive arcs appreciably more
in the monkey than in the frog. Its brunt falls, as we have seen
before, on the reactions of the skeletal musculature. Not that
in the highest animal forms the "distance-receptor" merely /^r
se has necessarily reached more perfection or more competence
than in the lower. In lower types, as in fish, are found
" distance-receptors " of high perfection, but their ablation does
not in lower types cripple in the same way as in higher types.
It is that in the higher types there is based upon the " distance-
receptors " a relatively enormous neural superstructure possess-
ing million-sided connections with multitudinous other nervous
arcs and representing untold potentialities for redistribution of
so-to-say stored stimuli by associative recall. The development
and elaboration of this internal nervous mechanism attached to
the organs of distance-reception has, so far as we can judge, far
outstripped progressive elaboration of the peripheral receptive
organs themselves. Adaptation and improvement would seem
to have been more precious assets in the former than in the
latter. And, as related to the former rather than to the latter,
must be regarded the parallelism of the ocular axes and the
overlapping of the uniocular fields of photo-reception which in
mammals has gradually reached its acme in the monkey and
in man. This overlapping yields, in virtue one would think of
some process akin to Herbart's ** complication," an important
additional datum for visual space. This, together with promo-
tion of the fore limb from a simple locomotor prop to a delicate

DC] THE CEREBRUM 353

explorer of space in manifold directions, together also with the
organization of mimetic movement to express thoughts by sounds,
have with the developments of central nervous function which
they connote and promote been probably the chief factors in
man's outstripping other competitors in progress toward that
aim which seems the universal goal of animal behavior, namely
to dominate more completely the environment Remembering
these conditions, it need not surprise us that the distance-recep-
tors more and more exert preponderant directive influence over
the whole nervous system. To say this is to say no more than
that the motile and consolidated individual is driven, g^uided,
and controlled by, above all organs, its cerebrum. The inte-
grating power of the nervous system has in fact in the higher
animal, more than in the lower, constructed from a mere collec-
tion of organs and segments a functional unity, an individual of
more perfected solidarity. We see that the distance-receptors
integrate the individual not merely because of the wide ramifl-
cation of their arcs to the eflector organs through the lower
centres ; they integrate especially because of their great con-
nections in the high cerebral centres. Briefly expressed, their
special potency is because they integrate the animal through its
brain. The cerebrum itself may be indeed regarded as the
ganglion of the distance-receptors.

354 SENSUAL FUSION [Lect.

LECTURE X

SENSUAL FUSION

Argument : Nervous integration in relation to bodily movement and to
sensation compared Sensual fusion in a relatively simple instance
of binocular vision. The rotating binocular lantern. Flicker sen-
sations generated at " corresponding retinal points " ; absence of
evidence of their summation or interference eidier with synchronous
or asynchronous flicker of similar frequency. Their interference
when the flicker is of dissimilar frequency. Talbot's law not ap-
plicable to "corresponding points." Fechner's paradox. Preva-
lence of contours under Weber's law and under binocular summation
compared. The physiological initial stages of the reaction generated
in either of a pair of corresponding retinal points proceeds without
touching the apparatus of the twin point. Only after the sensations
initiated from the right and left " points " have been elaborated so bi
as to be well amenable to introspection does interference between
the reactions of the two (right and left) eye-systems occur. The con-
vergence of nerve-paths from the right and left retinae respectively
toward one cerebral region is significant of union for co-ordination
of motor reaction rather than for synthesis of sensation. Resem-
blances between motor and sensual reactions. The cerebrum pre-
eminently the organ of and for the adaptation of reactions.

The animal whose nervous construction we have been attempt-
ing to follow thus far, we have supposed merely a puppet moved
by the external world in which it is immersed ; and we have sup-
posed it a puppet without passions, memory, feelings, sensations,
let alone ideas concrete or abstract. From time to time we have
purposely invoked appeal to sensations and feelings such as our
own experience of ourselves provides in order to see better
whither lead the blind reactions of the thing that we have been
imagining a fatal mechanism. Whether such sensations or feelings
accompany or do not accompany the reactions we have been
studying we have left open. We have tacitly consented that our
point of study of those reactions leaves that question, to which
the present time gives no clear answer, as one with which we are
not concerned. But we may agree that if such sensations and

X] REFLEX UNION 355

feelings or anything at all closely like them do accompany the
reactions we have studied, the neural machinery to whose working
they are adjunct lies not confined in the nervous arcs we have so
far traced but in fields of nervous apparatus that, though connected
with those arcs, lie beyond them, in the cerebral hemispheres.

In the attempt to trace the integrative work of the nervous
system on its motor side, one of our leading principles has been
that of the " correlation of reflexes iUfout a final common pathJ*
Owing to the convergence of many various reflex-arcs toward
and their confluence in a common efferent path co-ordination in
their use of that path obtains and is demonstrable.

It has been shown that some reflexes are so correlated in regard
to a final common path that their actions on it coalesce and rein-
force each other. These are allied reflexes and have allied arcs.
Good examples of allied reflexes and arcs are those which arise
in receptors of one species distributed in one regional locality and
subserving one and the same type-reflex ; such are the arcs from
the shoulder region of the dog subserving the scratch-reflex.
We have also seen that reflexes which use the same "final
common path " but use it to different or opposed effect are so
correlated in regard to it that one reflex can temporarily inhibit
the other from use of the path. These reflexes we termed in
regard to each other antagonistic.

From these motor reactions it is natural to attempt to cross
the gulf from movement to sensation. In the bulbo-spinal dog
we may produce a flexion of the fore limb by stigmatic stimula-
tion of the outer digit. A reflex in its motor expression to all
outward appearance like the preceding we may also provoke by
simultaneous stimulation of the skin of the innermost together
with that of the outermost digit Or we may evoke a similar
reflex in the limb by stimulating simultaneously with the fore
foot the opposite hind foot Here there is no conflict between
the reactions to the component stimuli. We may add further
the simultaneous stimulation of the same side pinna. The reflex
is then of more compound origin, but its component reflexes are
so correlated about their ** final common paths*' to the fore limb
that their actions there coalesce and reinforce. Further, we may

356 SENSUAL FUSION [LsscT.

add to ibese stimisU others ^plied to receptors of species actu-
ally different from any of the cutaneous and thus still finrtiier add
to the sources of the total reflex; and we may choose sources
which are harmonious and the impulses from them flow togedier
and combine.

On the other hand, instead of adding factors that tend to
combine in the production of a particular reflex we may excite
simultaneously with other sources a source whose reaction is
incompatible with theirs. Then struggle and rivalry ensue and
the result may be inhibition of that particular reflex movement
and appearance of some other.

It appears to follow from such considerations that when we
find electrical excitation of certain spots of the cerebral hemi-
sphere regularly evoke certain movements, $.g. of a limb, die
probabili^ is that we have there a nodal point where various
harmoniously acting neural arcs are tied together and can be
there reached and driven as a unit — though a highly syn-
thesized one — and produce the effect which is the common
resultant of them all.

The receptive points and organs which under stimulation
initiate reflex movements also initiate, in the intact animal with
unmutilated brain, sensations. As each reflex has a reflex action
attributive to it, so it has potentially at least a sensual reaction.
These sensual reactions, like the motor reflexes, are of various
grades of complexity. The simple perceptual image of an object
is usually a resultant as regards external stimulation of stimuli
applied jointly to several sense-organs. It has direct sensual
factors traceable from various sources. The cigar taken from its
box may be simultaneously sensed through eye and hand and nose
and ear. The object experientially regarded as a single object
excites a neural reaction that has its starting points in many
spatially and qualitatively distinct receptive points, each point
the commencement of separate nervous arcs. In the psychical
result of the reactions thus set going there is amplification and
modification by conditions memorial, affective, judicial, conative,
etc., obtaining in the mind and not due immediately to the stim-
ulus. The neural process resulting from the nervous impulses

X] BINOCULAR FUSION 357

initiated by the retinal, olfactory, cutaneous, and muscular recep-
tors is therefore internally modified in the nervous system by
processes and states already existent there or evoked there by
itself as a reverberation of its action. Can we at all compare
with the simultaneous co-ordination of the nervous factors in a
motor reflex the synthesis of the nervous elements whose com-
bination underlies a simple sense-perception?

We may somewhat reduce the complexity of the sense-
percept by limiting its paths of initiation to those of a single
sense, namely the visual, excluding the object, e.g. the seen
cigar, from directly stimulating other sense channels, tactual,
olfactory, auditory, muscular, etc. The cigar may be offered
only to the eye. Then we have left as regards the external
stimulation merely the fusion of the right-eye and the left-eye
images. This fusion is so complete that we cannot by intro-
spection discriminate in the visual image the right-eye image
from the left-eye image. Moreover, this fusion is so elemental
that introspection cannot detect in it any effort of memory,
judgment, or reason. It; appears innate — a datum ready pro-
vided even at dawn of individual human consciousness. We
can further strip the problem of some of its complexity by sub-
stituting for the three-dimensional object, e. g. the cigar, with
its perspective shading and its patches of colour and its char-
acteristic associations, etc., a simple and relatively meaningless
discoid patch of moderate, even, and uncoloured brightness,
small enough to lie wholly on the central area of each retina.
We can then test to what degree the visual singleness of the
observed surface, sensed through right eye and left eye together,
is due to direct confluence of the sensory paths excited by the
right-eye and left-eye images respectively. We can attempt
this in the following way.^^

A double sheet of thick milk-glass is observed by transmitted
light given by a lamp. This lamp is set in the axis of a rotating
cylinder (Fig. 76). In the side of the cylinder are three horizontal
rows of rectangular windows, tier above tier. The lamp, though
fixed in the axis of rotation of this revolving cylindrical screen,
is entirely free from all attachment to it The milk-glass plate

358

SENSUAL FUSION

[Lect.

o o

FlouRK 76. — Rotating Lantern. I. Elevation seen from front. IL Horizontal plan,
through level of A-A of I. Supports seen in perspective. The eyek>alls, pupil screens,
and convergant visual axes are indicated belonging to II, but carried through I. The
plan of the lantern is given one fourth actual size. Description in text.

THE ROTATING BINOCULAR LAMP

359

is fixed between the lamp and the inner face of the tiers of
windows, close to the latter.

Outside the moving cylindrical screen is a fixed semi-
cylindrical screen concentric with the revolving one» and just
wide enough to allow the inner revolving one to turn within it
freely (Fig. 76). In the fixed cylindrical screen four circular
holes are arranged so that two are centred on the same horizon-
tal line, and of the other two one is centred just so far above the
left-hand hole of the just mentioned pair as the other is below
the right-hand member of the pair. The horizontal distance
between the centres of the right and left hand holes is 9 mntL
The diameter of each hole is 8 mm. The vertical distance

w////////// ,

FiGumz 77.

between the centres of the holes is exactly the same as that be-
tween the centres of tiers of the revolving cylindrical screen,
namely 11 mm. These four circular holes in the outer fixed
cylindrical screen are, in the experiments, viewed from a distance
such that when the line of visual direction of the right eye
passes through the centre of the right hole it meets (Fig. 76)
at the axis of the cylindrical lantern the line of visual direction
of the left eye, which latter line passes through the centre of the
left-hand hole.

36b SENSUAL FUSION [Lbct.

This beit^ 9O9 the images of the lower Ieft4i2tfid hole and of
tiie upper right-hand hole fuse visually to sit^^eiiess. Thqr
then BpptBT as the middle one of three arranged vertically one
above the other.

A black vertical thin screen set at right-ai^^ to tiie plane
of the forehead is introduced (Fig. yy) between tiie eyes and the
holes so as to screen from die left eye all view of the right-hand
holes, and from the right eye all view of die left-hand holes.

The revolving screoi is driven by an electromotor. The
speed of revolution of this motor is controlled by a coarse ad-
justment and by a fine adjustment The qpeed of rotation of
die cylindrical screen is recorded by marking the completion
of each revolution of its spindle by an electromagnetic signal
writing on a travelling blackened sur&ce (Fig. yy). On die
same surface the time is recorded by a writingclock maridng
fifths of seconds.

The inner revolving screen by its revolution opens and shuts
alternately for equal periods the circular holes in the fixed outer
screen. The inner screen with its three tiers of windows is made
in three pieces, each containing one tier of the windows. The
piece containing the middle tier of openings is jointed in such a
way that its openings can be set at any desired interval with
the openings of the lowest tier. The highest tier is similarly
jointed to the middle tier. In this way it can be arranged that the
uppermost circular hole is open when the lower ones are closed,
or is shut when the lower are closed, or is opened to any desired
degree either before or after the lower; further, by removing
the top gallery of the rotating screen it can be left permanently
open. A similar relationship is also allowed between the middle
holes and the lower.

By wearing weak prisms with their base-apex lines vertical
the images of the right-hand and left-hand holes can be brought
to the same horizontal levels. The observer can then immedi-
ately fuse the four images to two by convergence. A horizontal
fine thread halving each of the two middle holes, and similar
but vertical threads halving the other two holes, serve to certify
binocular vision to the observer. When the four holes are all

X] BINOCULAR FLICKER 361

allowed to act thus under appropriate convergent binocular
gaze they are seen by the observer as two evenly lighted discs,
one vertically above the other, and each cut into quadrants by a
delicate black cross. By separately adjustable shutters any one,
or any vertically edged fraction of one, of the discs can be sep-
arately screened out of vision.

The object of the above arrangement is to attain the fol-
lowing conditions. Images accurately similar are received by
retinal areas fully visually conjugate. The areas are not only
of the so-called ** geometrical identity," but are at the time of
the observation in full binocular co-operation, owing to the con-
current convergence and accommodation. Extinction and illu-
mination of the images occur pari passu in the two eyes, i. e.
with like speed and in like direction. It can be synchronous or
of any time-sequence desired. That the speed shall be similar for
the two is insured by all the shutters being on the same spindle.

Each disc-shaped image will have on the retina a diameter
of about 570fi. That is, when foveal vision is directed upon it,
the image will occupy a practically rod-free area containing
about 2,800 cones. The direction of translation being the same
for all the shutters the bright images on the two retinae are, if
the shutters are set for simultaneous right and left images, com-
menced on ** identical " points of the two retinae, established
progressively along " identical " points, and finally extinguished
in like manner progressively along " identical " points. Or, con-
versely, if the shutters are set for accurately alternate right and
left images the screening off begins in one eye at a spot and
moment identical with those at which the turning on of the
image commences in the other eye ; so similarly it finishes.
With the speeds of revolution used for the observations the time
the shutter takes to expose or occlude completely each bright
disc, varies between .011" and .002". Error that might arise
on this score is avoided by the consensual direction of move-
ment of the right and left hand shutters.

That the " retinal points " to which the images are thus ap-
plied synchronously or in desired sequence are truly " identical "
is certified, (i) by the paired physical images being seen single;

362 SENSUAL FUSION [Lbct.

(2) by the maximum di^Muatioo of the edges of the loCatiiig
shutters being about //& on the retina, whereas 35Q^ is about
the vertical retinal disparation which limits binocolar combi-
nation. Moreover, a contour travelling through a visual ai^
of 2^ in -^'\ as in these observations, is not percqytible as a
contour at alL

Difficulties due to change in pupil-width are excluded bjr
artificial pupils. Equality of brightness of illumination of the
four milk-|^ass-backed 8mm. holes b obtained by making the
straight-wire candle-shaped lamp of considerable, i. e. 12 cm.
length, and fixing it accurately in the axis of the cylindrical
screen. The rotating screen is blackened inside to minimize
reflection.

In this way two haploscopic images, one close above the
other, are placed in the central field. The right and left
components of each of these can be either synchronously or
alternately compounded in each. The foveal gaze can be
turned from one to the other of them when and as often as
the observer desires, and in the fraction of a second by a slight,
i. e. less than 3**, movement of the eyeballs. The comparison
thus instituted is facile and sure.

A. SYMMETRICAL FLICKER

With the apparatus thus arranged various binocular com-
binations can be investigated and compared either one with
another or with uniocular images.

As shown above, the apparatus allows of similar images
being thrown on strictly and fully conjugate points of the
two retinae, either synchronously right and left or alternately
right and left, with a time accuracy not less than .0006" for
the slowest rates of intermission, and not less than .0001" for the
highest. The first comparison made (Experiment i) may be
to observe if there is any difference between the rates of inter-
mittence for just perceptible flicker in two binocular images,
one made with synchronous right and left illuminations, the
other with alternate right and left illuminations. This arrange-

X} SYMMETRICAL BINOCULAR FLICKER 363

ment is expressed graphically by the accompanying diagram
(Fig. 78).

The diagram makes the lower composite image the "syn-
chronous " one, but in the series of observations the ** synchro-
nous" is sometimes the lower, sometimes the upper, and the
observer is not informed which it may be. The observations
may be made on the transition from flickering to unflickering
sensation, or conversely on the transition from unflickering to

.^i_j

_^ I

FiGumz 78.

flickering sensation ; the observer in the latter has a more neutral
approach to the critical observation. Compared under rates of
intermittence giving marked flicker in both images, observers
find the flicker " less " in the " alternate " than in the " synchro-
nous " combination. This diflerence at the lower speeds inclines
the observer to expect that complete extinction of the flicker
will disappear the more readily in the image which at slow inter-
missions seems to flicker the less. Judgment is therefore best
asked under conditions in which both images start perfectly
free from flicker, the rate of intermittence being from the outset
high enough to exclude flicker.

The judgment then given is almost uniformly that there
does exist a very small difference between the frequency of
intermittence required for extinction of flicker in the "syn-
chronous " and " alternate " combinations respectively. In the
" alternate '' combination flicker disappears at a slightly lower
frequency of intermission than in the "synchronous." All
observers agree that directly the frequency of intermission
extinguishes flicker in both the discs the appearance of both

364 SENSUAL FUSION [Lbct.

IS indtstiagoisliably aimilar» and that there is then nodiing to
choose between the brightness of the two.

For almost all persons I have examined* a spot intermit-
tently illuminated at a frequency of intermission just sufficient
to extinguish flicker in it, when looked at with one eye only,
still flickers slightly when looked at with both eyes. A like
phenomenon is noticed by most observers when examined by
the arrangement (Experiment 2) represented by Figure 791

^^ 1

PlGURB 79.

)

The binocular arrangement, then, is said by them to require
a slightly higher frequency for extinction of flicker than does
the uniocular. Again, if under a frequency of intermission

just securing extinction of flicker in either of the component
uniocular images separately, one of these images, previously
screened off, is readmitted, so that the pair act together with
a synchronous arrangement of phase, a trace of flicker appears
at once in the binocular image. It may be urged that this is
due to the fresh retinal area being more sensitive to flicker, and
it is true that the flicker so introduced tends soon to become
less, but a residuum of the phenomenon seems to remain.

Experiment 3. Conversely, under the arrangement indicated in
Figure 80, a number of the persons examined, but not all, decide
that the binocular image requires for extinction of flicker a slightly
lower frequency of intermittent Illumination than does the uniocular.
Also, a number of these persons, though not all, find that when the
" alternate right and left " combination is observed under a frequency
of intermission of illumination just sufficient to extinguish its flicker.

XJ SYMMETRICAL BINOCULAR FLICKER 365

•>L_^i__^

FiGURX 80.

the screening oat of one of the component aniocolar images brings
with it a slight appearance of flicker.

From these observations it appears that similar phases of flicker-
ing illumination if timed to fiall coinddently^ on conjugate retinal areas
do very sUghUy reinforce each other in sensation, and if timed exacdy
alternately do very slightly mutually reduce. But the broad outcome
of tiu observations is that so far from bright phases at one eye eflacing
dark phases at the corresponding spot of the other eye, 0ure is hardly
a trace of any such interference. To judge from its absence of influence
on the flicker rate, the dark phase incident at retinal point A' does not,
as regards sensual result, modify the bright phase synchronously inci-
dent at the conjugal retinal point A, and conversely. If the brightness
of the bright phase or the darkness of the dark phase were lessened at
A by A^ the rate of frequency of stimulus for extinction of flicker must
6dL But except in minute and perhaps equivocal degree it does not
alter.

As far as sensual eflfect goes, the light phases at the one eye
practically do not, therefore, interfere or combine with the co-
incident dark phases at the other; and conversely. Nor do
they, in the alternate left and right arrangement, add themselves
as a series of additional stimuli to the like series of stimuli
applied at the other eye. If they did the revolution rate of
the cylindrical shutter required for extinction of flicker in the
upper binocular range LR, Fig. 80, would fall far below that
required for extinction in the uniocular. This it does not do.
It does not fall at all, apart from the minute diflerence noted by
some persons as mentioned above. A similar result is obtained

366 SENSUAL FUSION [Lsci^

under die, in some wvys more dectstve, conditioiis (Ezpeiiraeat
4) represented in Fig. 81.

With this arrangement no observer in my experiments has
ever widi certainty detected difference between tbe uniocvlar
and binocular images in regard to either the iq[iparent nte of

FlGUKB 81.

the flicker when moderately coarse or the rate of intermbsioii
required for flicker extinction. This arrangement (Fig. 81)
seems the most crucial for deciding the point In the ** alter-
nate right and left'' arrangement (Fig. 78, LR, upper com-
bination) the instants of change of phase £adling together right
or left, it might be that it did not matter as regards f&cka
sensation whether the direction of change was from light to dark
or dark to light ; the rates of intermission being the same right
and left, and the instants of their incidence being synchronous,
it might then be that as regards flicker the arrangement was
only tantamount to the •* synchronous right and left " arrange-
ment (Fig. 78, lower combination) or to the uniocular inter-
mittence of the same rate. The arrangement (Fig. 81) avoids
this dilemma. Moreover it avoids both the minute reinforce-
ment and the minute reduction of flicker inherent, according
to the above experience, in the exactly " synchronous " and
" alternate " arrangements. It may be termed for conven-
ience of reference the " intermediate " arrangement The physi-
ological stimulation it delivers to the conjugate retina is by
any mode of count delivered at twice the rate of delivery for
either retina considered apart from its fellow. Yet the rate of

X] SYMMETRICAL BINOCULAR FLICKER 367

revolution of the cylindrical lantern required to extinguish flicker
in this experiment remains for the binocular image the same cu
for the uniocular.

There arises the question whether we may regard the dark
field covering the area correspondent with that to which in the
other retina a bright image is presented, as non-existent visually.
That assumption has been made above, and is indicated in the
diagrams (Figs. 79, 80, 81). In them, where one image is repre-
sented as uniocular, the conjugate area of the other retina is
left out of the diagram altogether, as though the latter retina
were non-existent, or for the time being blind. This seems per-
missible, when care is taken to insure absence of all detail or
contour from the dark field presented to the other retina, except
for the one component of the compared binocular image. When
that field is perfectly void of other contours, and unchanging
and borderless, it is found to matter little what depth of darkness
it has ; it may be a shade of gray or even a fair white, without
perceptibly influencing the sensual vibrations given by the flick-
ering image before the other eye. The condition seems com-
parable with the familiar disability to see the dark field presented
to one closed eye, when with the other eye the observer regards
a detailed image.^ For these reasons the visual image result-
ing from the presentation of the bright disc to one eye only,
as in the arrangements shown by Figs. 79, 80, 81, was regarded
as being a truly uniocular product, uncomplicated by any com-
ponent from the other retina. The corresponding area of this
latter was considered as for the time being out of action as
regards sense, not so much by darkness as by virtue of border-
less void homogeneity of field, — as when eye-closure affords
visual rest Under this blankness the " retinal points " become
unhitched firom the running machinery of consciousness, if —
and this is essential — the ** corresponding" retinal area be con-
currently under stimulation by a defined image. McDougall's ^^
principle of competition for energy between associate neurones
seems at work here, for with both eyes shut the dark blankness
of eye-closure does become visible. Even with one eye open,
if its field be undetailed and homogeneous, glimpses of the

Stt SENSUAL FUSION [Ua.

Iiwan^ i>f a dkmtd eye become obbdnabfe (PBrknic^
VoMcmann, E. Hering).

The mccanSbdy ccmvene stinmlatioii of tiie twin letiiial aieis
might be ex pected to give some tnterference of the flideer soi-
sationt so g e nerate d . But tint experimental evUence indicates
absence (practtcally entire) of any interference between the flidKr
p r o ce ss e s so initiated. The right and left '' correspon^ag
retino-ceiebral points '' do not when tested by flicker reactkais
behave as thongh combined or conjugate to a sin|^e mechanisaL
Their sensual reactions retain individuality as r^;ards time-rdar
tions even when completely confluent as judged by reference to
visual space.

& ASYMMETRICAL FUCKER

In the foregoing experiments the flicker sensations of *' cor-
responding*' areas of the two retinae appear (almost entirely)
without influence one upon another. But in other experiments
the flicker test reveab very considerable mutual influence between
reactions initiated at die corresponding areas.

"^i_^i_j

"M.^lMMJ

K©

FiGumK 8«.

Suppose (Experiment 5) two binocular images LR and \p
similarly combined from similar uniocular components, all indi-
vidually equal in brightness and in intermission frequency.
Suppose that of the components of one pair (Xp) one (p) be
replaced (Fig. 82) by an intermittent uniocular image (z^), of
the same physical brightness as that giving the visual image p,
but of considerably higher intermission frequency. In /^ all

X] ASYMMETRICAL BINOCULAR FLICKER 369

flicker will disappear at slower speeds of revolution of the lan-
tern than those required to extinguish flicker in L or R or X.
Fig. 82 represents the arrangement.

The frequency of intermission required to extinguish flicker
in \p is then found to be much lower than the frequency required
for extinction of flicker in LR, or in L or R or X separately.
Thus the frequency for extinction of flicker in \p was found
(observer H. H.) to average 52.2 phases per second as against
61.9 phases per second for LR, or for L, R, or X separately.

Screening image p' out of the binocular combination \p',
when the frequency of intermission was just high enough to
free the \p' image from flicker, at once brought flicker into it ;
this disappeared immediately image p' was readmitted to the
combination.

In this instance the intensity chosen for the steady illumina-
tion of the conjugate area was equal to that employed for the

PlGURB 83.

uniocular flickering image. The duration of the light phases
and the dark per revolution of the lantern was equal, and the
light and dark phases of the same intensity in both. But the
phenomenon obtains also when the steady uniocular image is
less bright (Experiment 6, Fig. 83) or more bright (Experi-
ment 7, Fig. 84) than the flickering uniocular with which it is
combined. The following example illustrates this.

Experiment 6, In the balanced pair of binocular images LR and
Xp made of carefully equalized intermittent uniocular images L, R, X,
and p the tmiocular image p was replaced by one f^ of five times greater

|Lke,

; a MMdlf iM«e of odj ^.Oe l]i«M-
lilfiM^* The ftB^uciMjjf of imfffii^

t tfie tnoodviniaBe V (^ 83)

I to be 7sa pfanes per teooiid, whereas in LK, and a

ft k wa» 75*5 pfanes per second as k had been k

< fr uieiuove uasBped
ivi IbB ^KBcnpf aeBBBDOB IMNU Hie rffwmgsif ^oc t
?XtraaBBiO lwpn S i l id by 3-4 phases peri

ry (F%>>4) Hsilislf an obse t i alka i in wfaidi far the
^kidie h sa ncnhf mashinstinn V an hnage ^ was snbstJtnted
01 MieeiSBgsai^gpwriieqpenqf oig uciin i iMnn anogivingastcsa^
of OM ifth cRster bq^hbsess dMB die nH«e L, R, and X when steady.
It was dien fbond that tiie frcqnenqr of intennlssion required to eztin-
goish flicker in the binocular image A^ (Fig. 84) was 57.8 alternate
equal phases (of X) per second. Whereas in LR, and in X, L» and R,
taken separately, the number of such phases required nas 63.6 per
second.

The image Xf/ was distincdy brighter visually than was LR,
or any of the uniocular images X, L« and R.

These observations show, as do the observations represented
by Fig. 82 » that it is not merely the reduction of brightness in the
combined image \p in the arrangement shown by Fig. 83 that
lessens the flicker in the latter. In tad^ the observations on the
plan illustrated by Fig. 84 we have the, for flicker photometry,
interesting case of a brighter intermittently illuminated sur£sice
flickering less than a duller one.

Here the conditions of experiment suggest that the addition

X] UNICX:ULAR AND BINOCULAR 371

of the steady brightness at one eye to the dark phase of the
intermittent at the *' corresponding spot " lighten the latter, and
its addition to the phase of equal brightness with it leave that
practically unaltered.

That might be evidence of mutual interference between
purely phs^siological processes initiated at the corresponding
spots of the right and left retinae. But, on the other hand, the .
result at once suggests that the binocular product from a uni-
ocular flickering and 9 uniocular unflickering image arises by a
synthetic process akin to that which produces from a pair of
individual uniocular brightnesses a binocular brightness near
the arithmetic mean of the brightness of the two components.
The rule of combination exemplified by these latter finds little
solution by appeal to summation or interference of retinal and
purely physiological processes.

Moreover, the supposition that the sensual reaction caused
by a steady image acting at one of the pair of " corresponding "
areas, is interfering with or combining with the individual phases
of reaction to the intermittent image at the fellow area, is exactly
the supposition that the observations dealing with symmetrical
flicker show to be untenable.

UNIOCULAR AND BINOCULAR COMPARISONS

With intermittent lights throughout a wide range of ordinary
intensities Talbot's ^^ law is unimpeachable for the single eye ;
and also for the two eyes if employed together under, as is usual,
arrangements practically equivalent to the *' simultaneous " right-
left method for " symmetrical flicker." It is interesting to dis-
cover how far the double retina will still observe Talbot's law
when subjected to treatment such that, if the retina did then ob-
serve the law, would indicate its integration to a functionally
single retina. In other words, under a rapidly repeated stimu-
lus, when one incidence of that stimulus has acted on a retinal
point the question is: how far is it the same thing for visual
brightness, whether the next incidence be upon the same retinal
point or upon the twin point in the other retina? How far can

372 SENSUAL FUSION [Lect.

the double retina, when functioning for singleness of perception
in binocular vision, be considered as functionally combined to
a single retina, and how far does it then react as does a single
retina, if examined for Talbot's law?

The " alternate left-right arrangement" (Experiment i, LR)
supplies the required method of stimulation. With speeds
of revolution of the lantern too high to allow flickering, the
binocular image LR (Fig. 79) is seen to appear of brightness
equal to X/>, and with the uniocular images X or f> taken singly.
Therefore in the above sense, Talbot's law not only does not hold
for the double retina considered as functionally single, but no
trace of observance of the law is detectable. The two corres-
ponding points are therefore in this respect not integrated to
a single retinal surface.

It was often noted that with all four lantern images of equal
luminosity, using intermission frequencies too rapid to allow
flicker, the brightness of the binocular combination of any two
did not distinctly exceed that of the uniocular. In certain in-
stances the binocular combination did appear just distinctly the
brighter. This was for instance the case when of the four lan-
tern images the two on the same horizontal level were combined
by simple convergence. This excess of brightness is the well-
known phenomenon examined by Jurin,^ Harris,® Fechner,*
Aubert,®" Valerius,^^ and others. But there occurred frequent
instances in which no excess was observed in the brightness of
binocular combinations over that of their carefully balanced
uniocular components. In these observations the brightness of
the physical images is however always much above the threshold
of the light adapted eye; and I have not made systematic obser-
vations with the eye dark-adapted. To obtain good conditions
for comparison of the brightness of the binocular and uniocular
images the following arrangement can be employed.

Experiment 8. Two images LR and ApJ are placed in the visual
field for mutual comparison. LR is composed of left-eye and right-eye
equal and corresponding disc-shaped images as in previous experiments.
ApJ is composed of a left-eye image similar to L and R except that it
lies just above or below them in the visual field. With X*s right half

X] TALBOT'S LAW AND BINOCULAR RETINA 373

is combined the image of the right half of a lantern image similar again
to the others, except that its left half is screened absolutely off into the
blank undetailed darkness of the general field. When this is done the
two opposite visual images LR and Xp\ regarded under perfectly steady
ocular fixation are stable, and no difference of brightness is discernible
between them. Moreover no join is seen between the halves of Ap^ and
no difference of brightness between the halves. After prolonged in-
spection of them rivalry becomes troublesome ; but a judgment can be
clearly arrived at before that happens.

In this experiment it might possibly be that equality of
brightness between the halves of \pi is due to image pi not

Exp. 9.

Figure 85.

really being in consciousness at all during the comparison. The
image might possibly lapse under competition with the partly
dissimilar correspondingly placed left-eye image X. Experi-
ments carried out by W. McDougall^^ give validity to such
possible objection. The perceptibility of the horizontal bar in
the right half of the image \p\ is guarantee however that at
least part of the uniocular image p^ is present. But to ascer-
tain more surely whether image p\ is really during the visual
equation co-operating in consciousness with X the following
further arrangement can be employed.

Experiment^ (Fig. 85). With the revolving lantern so arranged
.that images L, R, X, and p^ are all of equal brightness when steady and
nnflickeringy p^ is given at a lesser frequency of intermission, so as to
flicker while the others do not. A speed of revolution of lantern is then
iiaed ac which just a trace of flicker is perceptible in p^ when binocularly

374 SENSUAL FUSTON JUa.

combtoed widi A. The e^oatite Ut » A|»| k then ficMiiid to IkM
flicker is itiU just tmceable in the i^ht half of A^ There it then no
jmn seen between the hahres of Xp|nor aiqr diflj^rence betiveai the
bng^itneai of the halves. So loQg as ocular fixation is steady no liialqr
disturbs the observadon.

In this case there can» I think, be no question bat that the
one half of 'kpi is truly binocular, for the trace of flicker is
perceptible during the actual performance of the comparisoa.
Yet no difference of brightness is perceived between LR and
'kpit and the two lateral halves of Xpi compared together
seem of like brightness.

Even when the binocular image does show the well-known
slight excess of brightness over its uniocular components it,
under some conditions {v. sufm, ''alternate" arrangementX
flickers no more or even less than they.

It is doubtful therefore to me whether the slight excess in
brightness of the binocular image over its two equal uniocular
components is really explicable as summation of the intendties
of the reactions at the corresponding spots of the two retinae.
Valerius" measured the increase to be one fifteenth of the
brightness of the uniocular ims^;e. Aubert's'' diagram gives it
as less than one thirtieth. Aubert says it is not perceptible
with brightness greater than that of white paper in diffuse day-
light indoors.®^

In certain modes of experiment a uniocular image used as
a standard for comparison might itself be suffering some reduc-
tion in brightness owing to slight combination with the dark
field presented concurrently at the corresponding retinal area.
But •' rivalry " should reveal such influence. A better definition
and greater vividness of detail assured by better accommodation
and convergence under binocular regard, might possibly give
an appearance of greater brilliance and intensity. But these are
only suggestions.

I conclude that, with ordinary intensities of illumination,
although a binocular image does sometimes appear of slightly
greatervisual brightness than either of two similar uniocular images
composing it, more often it has a visual brightness not perceptibly

X] UNIOCULAR AND BINOCULAR BRIGHTNESS 375

different from that of either of its two co-<quaI uniocular com-
ponents. The case then falls within a general rule regarding binoc-
ular brightness attested by all observations I have on that subject.^
A binocular brightness compared with its uniocular components is of
value not greater than the grecUer of those ^ nor less than the lesser
of them; when free from oscillations of rivalry its value is
somewhat^ but not far^ above the arithmetic mean of the values
of the two uniocular components as expressed by the measures of the
physical stimuli yielding them.

The various combinations cited in Experiments 2, 3, 4, 5, 6,
7» and 8 have all, when steady and unflickering, given bright-
nesses illustrating the above rule. Other illustrations are

X = 1000, p = 250, \p = 680,

X=IOOO, p=: 350, Xf)= 750,

X=iooo, />= ssc \p= 83s,

X = 1000, p = 750, \p = 920,

X = 1000, p = 1000, \p = 1000.

But I have not worked with combinations where the phsrsical
luminosity of one uniocular component has been less than -j\|th
the physical luminosity of the other. It was near this limit that
Aubert, and just beyond it that Fechner, noted decline of the
darkening effect of the darker component In my own few
observations beyond that point the oscillations of rivalry have
made judgment difficult. The more manageable examples are
but demonstrations of '' Fechner's paradox," and fall under the
above general rule. Hering^ has suggested that rivalry is
really occurring even with similar and right and left uniocular
images; he says these react according to a law of "comple-
mental shares/' and offers a theory, such as the name he gives
implies, in explanation of the phenomenon. My Experiment 9
seems to offer difficulty to such a view.

Binocular combination of a less bright image with a more
bright gives a visual image of less brightness than the latter
(as stated in the rule above). But the application of the less
bright physical image to the same uniocular area as the more
bright gives a visual image of brightness greater than either.

376 SENSUAL FUSION [Lbct.

As above described, a steady image presented on an area of
one retina '' damps " the flicker of a flickering inu^e concor-
rently presented at the corresponding area of the other. A
steady ima^e actually physically superposed on the same retinal
area as a flickering one also reduces the latter's flicker: this
latter is of course in accordance with Webei's law. The modes
of interference seem incomparably different in the two cases ;
and experiment shows that the two interferences are often quite
of different value.

Experiment JO A. Binocular fusion of R and L gives flicker ex-
tinction at 65.5 phases per second.

Physical fusion of R and L j^ves flicker extinction at 59.4 phases
per second. Observer G. C

Binocular fusion of R and L gives flicker extinction at 106.6 phases
per second.

Physical fusion of R and L gives flicker extinction at X00.3 phases
per second.

R separately gives flicker extinction at 1x3.3 pb^^aes per second.

Observer R. S. W.

Finally, to touch on '* predominance of contours.'' Its facts,
established by so many workers, are among the most signifi*
cant concerning the difference between binocular and uniocular

fusion of visual reactions. I will merely give one illustration
which seems specially instructive for the point before us.

Experiment 11. A steady unflickering disc-shaped image L is pres-
ent to the left eye : across the disc is a narrow dark line. An image R
of similar size and shape but without the dark line is presented to the
corresponding area of the right eye. If the luminosity of L is progres-
sively diminished, a value of luminosity is reached at which its cross-
line, though visible when L alone is observed {e. g. right eye closed)
is lost or uncertain in the binocular image RL. This reduction of the
luminosity of L much exceeds the reduction at which its cross-line is
lost when image R is concurrently thrown on the same area of the
same retina, /. e, left retina. Thus, in one experiment the diminution
of luminosity of L required for loss of the cross-line under the physical
superposition of R and L on the same retina was 84 per cent, while
the diminution of luminosity of L required for loss (or great uncer-
tainty) of the line in the binocular image was 96 per cent

X] UNIOCULAR AND BINOCULAR CONTRAST 377

Not only the ease, but the mode of disappearance of the
cross-line, is significantly different in the two cases. In the
'* physical superposition" the dark line became gradually
thinner and fainter, and finally imperceptible, as the image L
is lessened in luminosity. In the case of binocular fusion the
dark line oscillates out of and back into sensation- more and
more, the disappearances predominating more and more as the
darkening of L proceeds. At a reduction of 84 per cent of
the luminosity of L the cross-line was steady, dark, and sharp
in the binocular image.

Our aim has been information as to the nature of the con-
junction between the uniocular components in certain simple
binocular sensations. The question concerns the nature of the
tie between "corresponding retinal points," meaning by "retinal
point" the retino-cerebral apparatus engaged in elaborating a
sensation in response to excitation of a unit area of retinal
surface.

That a sensation initiated from corresponding retinal points
is commonly referred without ambiguity to a single locus in
visual space has often been regarded (Newton,* Wollaston, ^'
Rohault, ' Joh. Miiller**) as evidence of community of the nerve
apparatus belonging to the paired retinal points. Their visual
image appears single. Wollaston supposed the twin points
attached to one and the same nerve-fibre, which bifurcated at
the chiasma. Rohault and Miiller supposed the points to be
served by twin fibres " from one and the same ganglion-cell in
the cerebral substance." Later (cf. Aubert*^), the visual single-
ness, spatial fusion of right and left impressions to a single per-
ception, was taken to mean confluence of the nerve-processes,
started in right and left retinae respectively, to " a single
common centre or point of the sensoriura." The discovery
later still that the fibre-tracts from corresponding halves of the
retinae both go to the occipital region of one and the same
hemisphere has also been inferred to mean a spatially conjoint
visual sensor! um common to both retinae {e, g. Ramon-y-Cajal,
Schafer). But in such questions the inferences obtainable from

378 SENSUAL FUSION (Lbct.

mere anatomical featntes are equivocal and often lemole ia
bearing. Were tbere to exist snch a common medumism aita*
ate aa a unit at conjunction of Ibe two convergent ^sterns and
were phases of excitement timed so to arrive from one retina as
exactly to fill pauses between excitations transmitted from tte
other» thenrthoe should be evidence of this in the time-cdatioBS
of the phenomena induced. The state of excitement should tend
to be maintained across periods that would otherwise cbecker it
as pauses.

The retino-cerebral apparatus may be re|[aided as a aliuctm e
of linked branching nerve-elements forming a system which ex-
pands as traced centrally from the retinal surface. It masy be
figured as a tree, with its stem at the retina and an arbcmzation
spreading into the brain, its ramifications there penetrating a
vast cerebral field, interlacing with others in a cerebral forest
composed of nervous arborisations. The simile fails» because in
the nervous forest the arborisations make functional union one
with another. Is the fusion of the perceptions adjunct to paired
*' corresponding points'' the outcome of a close concrescence of
their neuronic or neuro-fibrillar arborizations, making of them
practically a single upgrowth common to twin (right and left)
stems rooted in the corresponding retinal units? If so, how low
down, how close to their origin, are the twin ss^stems grafted
together, giving structural community to all the superstructure?

In the chain of nerve-elements attached to a sense-organ we
infer in general that to the activities of the most peripheral links
fier se psychical events are not adjunct Psychical processes,
beginning with least complex and ascending toward develop-
ment through many grades, attach to the chain in such a way
that for the simplest only the more peripheral portions of the
chain need be connected with the sense-organ, while for the
more complex the central portions in addition become more
extensively involved. But in the higher reactions of definite
physical aspect, e, g. sense-perceptions, the lower apsycbic and
less definitely psychic activities are also implicate. Where
from two sense-organs, e. g. two units of retinal surface, the
two nerve-chain arborizations are mutually connected, so that

X] BINOCULAR FUSION 379

the lower activities of the one affect by low-level side connections
the elements forming the other, there analysis must fail to distin-
guish in the full reaction what higher components may be sepa-
rately referable to one only of the two individual chains. The
processes apsychic, or so indefinitely psychic as to bafHe intro-
spection, at root of those amenable to introspection, must by
their coalescence defeat attempt to trace the final psychical
product to either of its two possible sources, so long as both
sources are open for its origination.

Were the nervous reactions initiated at twin points of the ret-
inae early in its path along the retino-cerebral nerve-chain^ to
enter mechanisms common to both, there must, under " alter-
nate" or "synchronous" right-left arrangement of stimuli (Fig.
78), be interference, algebraic summation, etc., a coalescence of
events which, though apsychical in itself, would involve subse-
quent confusion together of the sense-reactions of the two eyes.
A state of things wholly different from this is revealed in the
results of our experiments. And it would amount to the same
thing whether two quickly successive flashes of a light fell both on
one and the same member of a pair of ** corresponding points,"
or whether the first fell upon one member, the second upon the
other. But the experiments show that the effect in the two
cases is widely different. Talbot's law is not applicable to the
double retina, that is, to the two retinae functioning together in
binocular vision. The experimental results go to disapprove
the existence of any such fusion or interference between the
apsychical or even the subperceptual events arising from corre-
sponding retinal points. At most they indicate hardly discernible
traces of such interference (Experiment i). They indicate, on
the contrary, that such simple forms of binocular perception as
have been dealt with here are themselves fusions of elaborated
uniocular sensations. Since left and right end-results emerge
pure, " hybridization " has not mixed the early stages in their
evolution.

But the difference between the modes of stimulation left and
right is a difference that, although it should be potent if the left
and n^X. physiological machinery were conjoined to unity, should

38o SENSUAL FUSION [Lect.

constitute no difference when left stimtdatton is compared widi
right Emulation by the perceptual product which each jfidds.
I The left eye and right eye flickering visual images, each viewed
\ singly, do NOT (apart from the faint cross-line for recognition)
I differ to introspection. If the senisations derived firom the left
eye and right eye respectively appear under introspection tadts>
tinguishably alike, what ground is there for mutual interference
between them? It is much as though, of the left and right lan-
tern images each were seen by one of two observers, with similar
vision, and as though the minds of the two observers were com-
bined to a single mind. It may be recalled that binocular unifi-
cation of images, as we possess it, seems a comparatively late
achievement of phylogenetic evolution.

When the visual product of the two retinae is thus regarded
it is not surprisii^ that Talbot's law fails for the binocular cjrdo-
pean retina. It fiails because the binocular sensation is a fiision
of uniocular sensation and from no two similar sensations can a
resultant sensation be compounded different from its com-
ponents. Were Talbot's law to hold in the above sense for the
binocular retina there would, under the *' alternate left-right
arrangement" (symmetrical flicker), at rates of intermission too
high for flicker, result from an image L of brightness 3c, and an
image R of similar brightness x, a combined image LR of
brightness x + x, the value of the summed brightness being in
accord with the Weber-Fechner rule of summation of sensual
intensities. But, as shown, not only does this summation not
occur, but nothing like it occurs. The binocular result most
often does not perceptibly differ from either of its two co-equal
components.

But the experiments with uniocular components dissimilarly
flickering, and with flickering components concurrent with
steady components, do evidence (unlike the other experiments)
interference between the two eyes. This result might be inter-
preted as the outcome of community of the physiological
mechanisms attaching to the paired ''corresponding retinal
points." But the other experiments negative the existence of
this community. And the explanation just offered for the

X] BINOCULAR FUSION 381

absence of interference in the other experiments will account for
the presence of interference in these. From two components
perceptibly differing between themselves in regard to some
quality {e.g, flicker) a single combined sensual quality is ob-
tained, intermediate between that of the two components taken
singly. If the perceptible difference, e, g. in flicker, between the
components is wide, the fusion is liable to phasic oscillations of
predominance of one or other component Where the difference
in flicker is wide, such " rivalry " between the right and left com-
ponents is in fact not unfrequently seen. One component may
at the height of its phase be alone perceptible at the focus of
attention, the other component being inhibited out of focal atten-
tion or even out of conscious vision altogether. The inference^
is that only after the sensations initiated from right and left
" corresponding points " have been elaborated, and have reached a
dignity and definiteness well amenable to introspection^ does inter-
ferefice between the recutions of the two {left and right) eye-systems
occur. The binocular sensation attained seems combined from
right and left uniocular sensations elaborated independently.

And in harmony with this view stands the evidence adduced
for the rule formulated regarding the relation of binocular to
uniocular brightness. Further, the difference between the sen-
sual result of superposition of two similar images upon one and
the same area of a single retina, and upon twin areas of the two
retinae, could hardly be so great as it is, did apsychical or sub-
sensual reactions underlying " brightness " combine or interfere
in the two retinal systems. The binocular combination must be
a synthesis of a left-eye with a right-eye sensation. Similarly,
the " prevalence of contours " in binocular vision, and the phe-
nomena of •* retinal rivalry," are explicable if each member of a
pair of corresponding points yields a sensual entity which, when
not widely dissimilar from that yielded by its twin point, fuses
with that to a binocular sensation. In ** retinal rivalry " we have
an involuntarily performed analysis of this sensual bicompound.
The binocular perception in that case breaks down, leaving
phasic periods of one or other of the simpler component sensa-
tions bare to inspection.

S83 SENSUAL FUSION fUct.

W. McDot^aUt*' in 4>ply>ng to ''retinal rivakjr" ud
^ pievalence of contoisrs ** hb principle of conqietitimi d inter-
related nerve-danenU for energy, also aignes a '* separataiess
of tihe vfaoal cortical areas for 1^ two eyes." He brings for-
ward striking experiments in evidence of diis. In <me d tkese
he *"* shows that an after4mage» left from excitation of one
retina, is more strongly revived by subsequent weak diflbse
exdtation of that same retina than of its fellow. More recend^,
in experiments proving reinforcement of visual sensations by
the activity of the ocular muscles, as evidenced by after4m^[€
observations, he ^^ shows that activity o( the intrinsic moades
of an eye sends up to the brain an influence, reinfordi^ the
activity of the cerebro-retinal tract of that eye, while it exerts
no such effect upon the corresponding tract of the othtt eye, or
exerts it in a minor degree only. With this separateness of tiifc
mechanisms, wherein are produced the sensations g e nerated ia
the two retinae, our results by a different line of eacperimentatioa
accord well.

The compounding together ot right and left images really
nonidentical but not widely dissimilar, is (Ftoum, Herii^) the
basis of visual ^ relative depth-perception." The compounding
of visual images partiy dissimilar — flickering with nnflickering —
seems a simpler case in the same category of synthetic actions.
In our flicker experiments the visual components do not differ
as to space-attributes, and their combination has therefore no
resultant differential space-attribute. But the synthesis gives in
each case a compromise between the components in regard to
the attribute wherein they do differ ; in the flicker experiments,
that is in regard to the sensual steadiness of the brightness.
This amounts to the same as the rule formulated above for
binocular combination of brightness of different intensities, but
steady.

r Our experiments show, therefore, that during binocular regard
of an objective image each uniocular mechanism develops inde-
pendent fy a sensual image of considerable completeness. The
singleness of the binocular perception results from union of these
elaborated uniocular sensations. The singleness is therrfore the

X] VISUAL FUSION AND NEURAL UNION 383

product of a synthesis that works with alrecufy elaborated sen^
saHons contemporaneously proceeding.

The cerebral seats of right-eye and left-eye visual images
are thus shown to be separate. Conductive paths no doubt
interconnect them, but are shown to be unnecessary for visual
unification of the two images. The unification of a sensation of
composite source is evidently associated with a neurone arrange-
ment different firom that which obtains in the synthesis of a
reflex movement by the convergence of the reflexes of allied
arcs upon its final common paths.

Here we seem to have therefore contemporaneity of itself
sufficing for sensual synthesis, without necessarily any spatial
fusion of the neural processes or mechanisms involved, 1. e.
without spatial confluence to a unit apparatus. As mentioned
above, W. McDougall's experiments on after-images lead to a
like conclusion. The foundation of new correspondences be-
tween retinal points in cases of squint (Tschermak) strengthen
the same view. McDougall has recently well summarized the
position. But it is one not generally admitted by physiologists
or psychologists. Ziehen *•*» writes : " Schon ph)^iologisch ist
die Verschmeltzung der beiden Netzhautbilder dadurch vor-
bereitet dass die Erregungen welche auf den linken Halften
beider Netzhaute auftreten, vermoge der eigenthiimlichen par-
tiellen Sehnerven-kreuzung zusammen in die rechte Grosshim
hemisphare gelangen, und umgekehrt" And this was the view
of Joh. M tiller and of Aubert, and is advanced on histological
grounds by Ramon-y-Cajal.

The results bear also on the production of sensual reactions
and states more complex than those of the examples taken.
Hartmann, as quoted by McDougall, writes: ^'Only because
one part of my brain has a direct communication with the other
b the consciousness of the two parts unified. Could we unite
the brains of two human beings by a path of communication
equivalent to cerebral fibres both would no longer have two but
one consciousness." There is no denying the extreme impor-
tance and the vast actual extent of the spatial conjunction of
cerebral elements by conductive channels in sensual and per-

384 SENSUAL FUSION [Lbct.

ceptual reactiQiis. Yet I cannot bnt tlunk that its limitless
postulation leads not so much to explanation of the high degree
of unity of the individual mind as to an ultimate £adlacy.wfai<A
Professor James has trenchantly termed that of " the pontifical
cell." Pure conjunction in time without necessarily cerebral
conjunction in space lies at the root of the solution of the prob-
lem of the unity of mind.

Since convergence of the conductors from correspondii^
halves of the retinae to the same field of brain-cortex does not
signify physiological conjunction of right and left sense-imf^es-
sions, can we decipher at all what it does mean? To do so does
not seem difficult, and displays strikingly the different v^oe and
directness of spatial union of conducting-padis for motor ^athe-
sis and for psychical respectively. In animals with overlapping
visual fields the lateral movements of the eyeball have a mutual
relation different from the ordinary relations of the movements
of a unilaterally placed organ, e. g. a limb. Especially is this
the case where the overlap of the visual fields is exten^e, e.g.
where the ocular axes are parallel The horizontal movements
of each eyeball are balanced about the primary line of vision of
the globus in its habitual resting attitude, that is, in man,
straight forward. That line sensually, as shown by introspective
experiment, lies in the median sagittal plane of the head (Her-
ing). Hence the term * Cyclopean ' has been applied to the
biunial eye of human binocular vision. Finding the median
vertical plane of sight of the resting eyeball to correspond with
the median sagittal plane of the body, we may assume that the
motor reflexes deal with the eyeball conformably with that;
otherwise there would be disaccord between the reflexes and
the sensations. Therefore we must in any general consideration
of the taxis of the lateral movements of the two eyeballs transfer
in thought each eyeball from its own actual sagittal plane to
the median sagittal plane of the head ; and this latter corre-
sponds in the resting position with the sagittal median plane of
the animal as a whole.

Each lateral muscle of the eyeball comes therefore to bear
to the median plane of the body the same relation as does a

X] SEMroECUSSATION OF OPTIC TRACTS 385

Kmb on one side of the body. Thus, the external rectus muscle
of the right eyeball bears the same lateral relation to the
median sagittal plane of the body as does the right arm. And
the internal rectus muscle of the left eyeball bears the same
lateral relation to the median sagittal plane of the body as does
the right arm. Now a general arrangement evident in the
cerebral cortex is that the taxis of muscles lying to right of
the median sagittal plane is entrusted to the left hemisphere,
and vice versa. It is in accord with this that in animals with
overlapping visual fields the horizontal movement of the eyes
to one side should for both eyeballs be represented in one and
the same hemisphere. And as a fact the conjugate movement
is found represented for both eyeballs together in each hemis-
phere. If we regard, and it was shown above that we may do
so, the median sagittal planes of both eyeballs as identical with
the median sagittal plane of the head, they are identical with
each other, and the scheme of cortical representation may be
expressed thus : a point in the right retina and its twin point in
the left demand each of them identical movement of the two
eyeballs when, apart from convergence, those points excite their
own replacement by the fovea (1. e. when initiating a gaze). It
is obvious that the paths from the visual cortex of each side to
the eyeball muscles — experiment shows such a path to exist —
is connected therefore with both the right and left twin points.
That is, it is a common path* The confluence of conductors from
the two retinae to the same cortical field, though not uniting their
retinal impressions, gives them access to a common efferent path
which both must use.* At entrance to every common path lies,
as shown before, a co-ordinating mechanism. A co-ordinative
mechanism is thus obtained. This ''common path" with its
bilateral twin origin impinges in its turn, directly or indirectly, on
the motor neurones for the lateral eye-muscles, the final common
patlis. We have therefore to alter such a scheme as that furnished
by Cajal by attaching his convergent paths to efferent paths, and

* Mott has likewise independently urged that the interpretation to be placed on
this convergence of paths is motor rather than sensory. Trans. Ophthalm. Soc, voL
25, p. di, 1905.

386

SENSUAL FUSION

[Lect.

by divesting their supposed nodal cortical point of its h}'po-
thetical powers as a sensual Deiis ex miuhinan And we thus
meet another instance of convergence of afferent paths leading
to motor synthesis^ but not, or only remotely, to sensual. Seen
in this light the gulf between sensation and movement looms
even wider than was allowed for in the tentative suppositions
which prompted the above experiments on flicker. fl

VVe are thus warned against any hasty conclusion that the
neural mechanisms which synthesize reflex movements illustrate
in their arrangement also those concerned where sensual fusion
is the phenomenon. But that does not invalidate a broad prac-
tical inference which study of the nervous system in regard to
motor reaction allows. This inference is that toward the solu-
tion of the problems of motor taxis help is obtainable by appeal
to characters evident in sensual reaction. This practical infer-
ence need not in the least involve any doctrinal attitude what-
ever toward the hypothesis of psycho-physical parallelism. It
may proceed quite apart from that. It simply insists on the
likeness of ner\*ous reactions expressed by muscular and other
effector-organs to reactions whose evidence is sensual. It in-
sists on this likeness being close and fundamental enough to
make each of the two classes of phenomena of use to the
student of the other. A number of excellent investigators^^
hold, on the opposite hand, that the study of the two should
proceed apart even more rigidly than they do at present Con-
fusion has» it is true, been caused in both by the loose appUca-*^|
tion of the terms of the one set of phenomena to the other. But
to disregard the many significant similarities which exist between
the two sets is, it seems to me, to throw away one of the best]
instruments for discovery in both. We saw how suggestive]
psychological data prove for classifying the various species]
of receptors considered as initiators of reflexes. The after*!
discharge of a nervous arc finds expression not only in reflex j
movement but in, for instance, a visual after-image. Centripetal
impulses from eye-muscles reinforce visual (i. e. extero-ceptor)
sensations (Macdougall) just as centripetal impulses from the
leg muscles reinforce reflex movement induced from the skin

X] DISTINCT FROM REFLEX UNION 387

(cxtero-ceptor) of the foot. The " immediate spinal induction"
exemplified by reflexes has a counterpart in visual irradiation.
Visual contrast, if translated into terms of reflex contraction,
bears close resemblance to " successive spinal induction." The
features of fatigue repeat themselves in both sets of phenomena.
Receptors which initiate reflex movements adapted in regard
to objects at a distance initiate as sense-organs sensations pro-
jected into circumambient sensual space. Receptors which
initiate reflex movements advantageous in regard to some
locus of the surface of the body itself, e. g. removal of irrita-
tion thence, initiate as sense-organs sensations referred to that
same locus. Instances might be multiplied, but they have risen
prominently in several of the foregoing lectures, and are suffi-
ciently before our minds now. A practical inference from them
is that physiology and psychology, instead of prosecuting their
studies, as some now recommend, more strictly apart one from
another than at present, will find it serviceable for each to give
to the results achieved by the other even closer heed than has
been customary hitherto.

Besides this similarity of time-relation and other features
between the physiological and the psychical signs of neural
activity, another link connects the psychological and the physi-
ological for the biologist. To the physiology of pure reflexes,
that is, reflexes devoid of psychical accompaniment so far
as introspection can discover, psychological interest neverthe-
less attaches, and on a very distinct ground. This ground of
connection is seen if inquiry is followed along the animal scale
in the direction from higher forms to lower rather than by the
usually more favorable reverse approach. This is partly because
we directly observe psychical phenonema by introspection only,
that is, only in ourselves ; and the facts discovered by introspec-
tion are applicable to other beings the more readily the more
those beings resemble ourselves, namely, are animals ranking
near to man.

Pure reflexes are admirably adapted to certain ends. They
are reactions which have long proved advantageous in the phy-
lum, of which the existent individual is a representative embodi-

nient. Perfected during the course of ages, they have during
that course attained a stability, a certainty, and an ease of per-
formance beside which the stability and facility of the most
ingrained habit acquired during an individual life is presumably
small. But theirs is of itself a machine-like fatality. Their
character in this stands revealed when the neural arcs which
execute them are separated, e, g, by transection of the spinal
cord, from the higher centres of the nervous system. They can
be checked, it is true, as we have seen, by collision with other
reflexes as ancestral and as fatally operative as themselves
(Lectures V and VI). To these ancient invariable reflexes, con-
sciousness, in the ordinary meaning of the term, is not adjunct.
The subject as active agent does not direct them and cannot '
introspect them. fl

Yet it is clear, in higher animals especially so, that reflexes^
are under control. Their intrinsic fatality lies under control by
higher centres unless their nervous arcs are sundered from ties
existing with those higher centres. In other words, the re-
actions of reflex-arcs are controllable by mechanisms to whose
activity consciousness is adjunct. By these higher centres, this
or that reflex can be checked, or released, or modified in its re*
action with such variety and seeming independence of external
stimuli that the existence of a spontaneous internal process ex-
pressed as ** will " is the naive inference drawn* Its spring of
action is not now our question ; its seat in the nervous system ^^
seems to correspond with that of processes of perceptual leveL ^|
It is urgently necessary for physiology to know how this con-
trol—volitional control — ^is operative upon reflexes, that is,
how it intrudes and makes its influence felt upon the running
of the reflex machinery. How is the cough, or eye-closure, or
the impulse to smile suppressed? How is the convergence of
the eyeballs, innately associate to visual fixation of a near ob-
ject, initiated voluntarily without recourse to fixation on an
object? Or how is the innate respiratory rhj^hm voluntarily
modified to meet the passing requirements of vocal utterance?
No exposition of the integrative action of the nervous system is
complete, even in outline, if this control is left without considera*

X] REFLEX AND VOLITIONAL ACTION 389

tion. Reflexes ordinarily outside its pale can by training be
brought within it The actor, it is asserted, can shed tears at
will, or blush or blanch. Occasional instances are recorded of
power to slow the rhythm of the heart at will ; others, of power
to suppress the reflex of swallowing when it has entered on its
pharyngeal stage. Volitional movement can certainly become
involuntary, and, conversely, involuntary movements can some-
times be brought under subjection to the will. From this subjec-
tion it is but a short step to acquisition of coordinations which
express themselves as movements newly acquired by the indi-
vidual« The controlling centres can pick out from an ancestrally
given motor reaction some one part of it, so as to isolate that as
a new separate movement, and by enhancement this can become
a skilled adapted act added to the powers of the individual. In
regard to the ring finger, the motor co-ordination ancestrally pro-
vided gives extension of that finger only in company with the
fingers on each side of it. We can soon train ourselves to lift
the ring-finger alone without the others. The ** will " dissociates
the ancestral co-ordination. Similarly we can acquire the power
to move a part which neither reflexly nor otherwise would seem
to come within the scope of our voluntary innervation, although
of course there must be motor nerve and muscle upon which
our innervation can operate. Thus, we can learn to retract
the pinna of the ear ; the movement is at first accompanied by
other iacial movement, but later with practice it becomes exe-
cutable without other facial movement. A new reaction and
co-ordination has been gained by the individual.

The transition from reflex action to volitional is not abrupt
and sharp. Familiar instances of individual acquisition of motor
co-ordination are furnished by the cases in which short, simple
movements, whether reflex or not, are by practice under voli-
tion combined into new sequences and become in time habitual
in the sense that though able to be directed they no longer re-
quire concentration of attention upon them for their execution.
As I write, my mind is not preoccupied with how my fingers form
the letters; my attention is fixed simply on the thought the
words express. But there was a time when the formation of the

390 SENSUAL FUSION [Lbct.

letters, as each one was wrtttea, woald have occiq^ed my «Adle
attetitjott.

Vc^tiooal control of reflexes is a question of conrnfiaa-
tion not eiqdiddy befinre os pcevioasfy in tbese lectures. Its
analysb has not indeed proceeded br. We mdy prettdat Aat
some extension of tiie same proc e sses ootfined in Lectnres V
and VI, as operative in simultaneous combination and in snc-
cesave comUnation of r efl e xe s , must be operative in diis oon>
troL There we saw r^exesmodifyii^ each other, and tiie more
complex reactions bdng built up from simpler and won re-
stricted ones. Some extension of tfie same process should, ia
view of our in fe re n ces rq^arding tfie nature of tiie dominance of
tiie brain (Lecture IX), appfy here abo.

It is significant diat, aMKM^ tibe r e fl exe s controlled are
so often unccmsdous, consciousness is adjunct to tiie centres
which exert tfie contrcd. A lHol<^;ist, Professor lioyd Moigui,
has urged that "tiie primary aim, object, and purpose of a»-
sciousness is controL Consciousness in a mere antomatDn is a
useless and unnecessary epiphenomenoo.**^ A somewhat similar
tiioui^t rose inddentalljr to our Vps in a previous lecture (Lec-
ture DC). The pleasure-pain acconqpaniment erf* reflexes has often
been interpreted as carrying Aatmeanii^. Certain it is that if we
study the process by which in ourselves this control over reflex
action is acquired by an individual, psychical factors loom lai^^e,
and more is known of them than of the purely physiological modus
operandi involved in the attainment of the control. Hence,
psychological studies have been more numerous than physio-
logical in this field. It is found that kinaesthetic sensations of
the movement to be acquired or controlled, though helpful, are
less important than the resident sensations from the part in its
" resting " state. These latter, with the power to focus atten-
tion upon them, appear, in a number of instances, to be a most
necessary condition for the acquirement of the control. And
in the monkey, voluntary control of a limb is largely lost when
the limb has been rendered apaesthetic.^*'

A biological inference arises at this point. We have admitted
* Introdacdon to Compantive Psychology, London, 1894, p. rSs.

X] NERVOUS ORGANS OF CONTROL 391

that the organs to which psychosis is adjunct, namely, the brain,
and especially in higher vertebrates the cerebral hemispheres,
supply the surest touchstone to rank in the scale of animal
creation. That is to admit, in other words, that development
of these organs constitutes, on the whole, the best criterion to
the success of an animal form in the competition which lies at
the root of animal evolution. These organs, we have just
seen, are the organs of nervous control ; and that control is ex-
ercised mainly in the perfecting and readjusting of manoeuvres
of ancient heritage. The way in which we ourselves acquire a
new skilled movement, the means by which we get more pre-
cission and speed in the use of a tool, the handling of an instru-
ment, or marksmanship with a weapon, is by a process of learning
in which nervous organs of control modify the activities of reflex
centres, themselves already perfected for other though kindred
actions. Our process of learning is accompanied by conscious
effort These nervous organs of control form, therefore, a special
instrument of adaptation and of readjustment of reaction to
better suit requirements which may be new. New adaptations
whence the individual may reap benefit are thus attained. The
more complex an organism, the more points of contact it has
with the environment, and the more frequently will it need
readjustment amid an environment of shifting relationships.
These nervous organs of control being organs of adjustment will
be more prominent the further the animal scale is followed up-
ward to its crowning species, man. And these organs which
give adjustability to the running of the reflex machinery, as
such, seem themselves — perhaps, by reason of their constant
relative newness — to be among the most plastic in the body.
In man and the species near him, these organs are most de-
veloped, and their mechanisms are cerebral. These cerebral
mechanisms constitute the clearest criterion of evolutional suc-
cess. In these types it is cerebral function which best compasses
that modification of old and that development of new reaction,
which perfects the adaptation of the individual to the environ-
ment. The relatively high development in man of this organ for
individual adjustment of reactions makes him the most successful

392 SENSUAL FUSION [Lect.

animal on earth's sor&ce at the present epoch. No donbt At
greater part of all Ais adjustment of reaction will; in his case* as
he stands now, come under intellectual activity. In him reason
enables the individual profitably to forecast the future* and to
act the more suitably to meet it» from memory of the past. Mere
experience can, however, apart from reason, mould nervous reac-
tions in so far as they are plastic. The '' bakHung*\ of a reflex
exhibits this faculty in germ. In the humble spheres of nervous
activity, such as alone fall mthin the scope of these lectures,
simple sensori-motor experience seems to count for more dian
reason in the actual process of acquiring new motor corordina-
tions. Of course reason, directing effort, counts in the selection
of the field of operation of motor experience. But the inei&cacy,
as a means to arrive at a new motor correlation, of instruction
merely verbal, or of ideas constructed without motor experience,
is common knowledge. To learn skating or racquets hy simple
cogitation or visual observation is, of course, imposable. Here
mere sensori-motor experience is more valuable than any course
of reasoning can be. Hence the training for a new skilled
motor manoeuvre must be simply ad hcc^ and is of itself no
training for another motor co-ordination, — apart from the well-
known mutual influence of training on qonmetrical parts of the
body. Yet, in high animal types, the connection between skilled
movements and the so-called ** motor " region of the cortex
cerebri, and the defect in these which injury of that region en-
tails, countenances the belief that the " experience " involved in
this training, though not rational, is cerebral. The compensation
of co-ordinative defects which the cerebrum accomplishes after
cerebellar or labyrinthine lesions, points to a similar conclusion.
And we must remember that though the mere sensori-motor
experience counts for so much in the mastering of a new move-
ment, they are perceptual, and in man rational, processes which
initiate, maintain, and guide effort toward acquirement of an act
which is new.

We thus, from the biological standpoint, see the cerebrum,
and especially the cerebral cortex, as the latest and highest ex-
pression of a nervous mechanism which may be described as the

X] THE CEREBRUM 393

organ of^ and for, the adaptation of nervous reactions. The cere-
bruni} built upon the distance-receptors and entrusted with reac-
tions which fall in an anticipatoiy interval so as to be precurrent
(Lect IX), comes, with its projicience of sensation and the
psychical powers unfolded from that germ of advantage, to be
the or^zxi par excellence for the readjustment and the perfecting
of the nervous reactions of the animal as a whole, so as to im-
prove and extend their suitability to, and advantage over, the
environment These adjustments, though not transmitted to
the offspring, yet in higher animals form the most potent in-
ternal condition for enabling the species to maintain and increase
in sum its dominance over the environment in which it is im-
mersed. A certain measure of such dominance is its ancestral
heritage; on this is based its innate right to success in the com-
petition for existence. But the factors and elements of that
competition change in detail as the history of the earth pro-
ceeds. The creature has to be partially readjusted if it is to
hold its own in the struggle. Only by continual modification
of its ancestral powers to suit the present can it fulfil that which
its destiny, if it is to succeed, requires from it as its life's purpose,
namely, the extension of its dominance over its environment
For this conquest its cerebrum is its best weapon. It is then
around the cerebrum, its physiological and psychological attri-
butes, that the main interest of biology niust ultimately turn.

1648.

1662.

1671.

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1677.

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1752.

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1768.

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1811.

II.

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12a.

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Biedermann, W. Pfliiger's Archiv, di.
Bonnier, P. Le sens des attitudes. Paris.
26

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INDEX

AcHnian^ 129, 314.
Action-current of nerve, 70.
Adaptation, 324.

the cereBrum and, 588, 589.
Adapted reactions, reflexes as, 235-269,

^ 306. 3^. 389.
Adequate stimulus, 12, 13, 91, 24c, 345.
Affective tone, 231, 255, 263, 2«^ 319,

^ 327, 330-333-
Afferent arc, 7, 55, 107.
After-discharge, 14, 26-35, 5i» 74. 102,

103. 288, 280, 386.
After-image, 35. 386.
Aipiasis saxicMa, 129, 314.
Albertoni, 272, 397.
Allbutt, Clifford, 72, 399.
Allied reflexes, 119, 120-135, 143, 168,

290 348, 3S5..356.
All-or-nothing prinaple, 71, 72.
Alternating reflexes, 144, 200-203.
AiH€iurnSt 129, 3x3.
Anaesthetics,; 14, 80. 81.
Anderson, Langley and, 38, 3^.
Anelective receptors, 227, 310^ 318.
Anger, 260, 261.
Anodon, 85.
Antagonistic muscles, 84, 200-202, 279-

284.
Antagonbtic reflexes, 135-149, 188-190,

203, 204, 229, 290, 322, 355.
Anthropoid apes, motor cortex of, 272-

279,288, 290.
Anthropomorphic interpretation, 236.
Anticipatory reactions, 326-333, 340.
Aorta, 99.
Apithy, 18, 232.
Arackm, 239.

Artkropoda, 41, 84. 239, 315.
Asphjrxia, 79, 80.

Associative recall, 331, 332, 352. •
Astacus, 85, 105, 238, 250, 295, 329.
AsteriaSf 2391
Atrophy, 247.
Atropin, 305.
Attention, 234.
Attitude, 302-304, 336-344, 35» » *nd see

" Postural reflexes."
Aubert, rj2, 374, 37> 377. 396.
Aoerbacn's plexus, 64.

Aureiia aurita, 169.
Autonomic system (Langley), 318.
Axis-cylinder, fluidity of, 10^ 17.
Axones, 41.

Bab&k, £^ 401.

Babuchin, 38, 396.

V. Baeyer, 14, 79, 40a

BagUoni. S., 67, 69, 71, 80, 107, 117, 132.

205, 401, 402.
"Bahnung." 14, 175-178. 184, 185.
Ballance, C, 272.
Barblets, 129, 322.
Barker, L. F., 400.

Barometic pressure and sensation, 339.
Bastian C, 247, 270, 396.
Bayliss,W., 113. 398.

and Starling, 3, 236, 312, 399.
BeagU, 330.
Beaunis, 396.
Bee, 2«>.
Beer, Th., 399.
Beevor, C, 272, 284, 398, 402.

and Horsley, 272, 276^ 278, 290, 398.
Behavior of animals, 237.
Bell, Gh 38, ^70, 287, 395-
Bell-Maffendie law, 38, 79.
Belmondo, 142, 398.
Bergmann, 79.
Bernard, C^ 107.

Bethe, A.. 14, 18, 39, 41, 42, 63, 82, 118,
« .,238.250.399.401,402.
Bezold, A., 398.
Bichat, 256, 305.
Bickel, A., 238^99, 40a
Biedermann, W., 9, 30, 71, 79, 85, 105,

182, 400.
Bile-duct, 11.
Binocular brightness compared with uni-

ocular, 371, 374-375-
Binocular contrast compared with uni-

ocular, 376-377.
Binocular flicker,.357-383 ; symmetrical,
162-368; asymmetrical, 368-371.
Binocular fusion, 357-385.
du Bois-Reymond, 17, 3$.

R., 400.
Bonnier, P., 340. 342, 400, 401.
Bottazzi, F., 402.

INDEX

TsB^,

jf-**

"5-^49^ *» 3K^ 3#^

M^ I44» I4S» <33» 34^ 3SS

>3H»347»1S^

INycholpiii. 307.

M4. woo, J^i-

331-
of

»^
genesis of. 519k

uihLtbitonr ottliaeBce o^ aSi>>39a
iaterT«l2tkxi of the two beaifipi«res>

motor reactraos oC. cooiDajrwi vttk

spinal 299^29:5, y?5-506k
reittforciof influence o£» 175^ 17^
"silent" Selds o4 27S* 279^
nncqn^ motor xeprcscBtitioa iB»

Cerebral sulci. Tauriabfiitr of, 27^-176.

not fonctiooal booBaaries,
Cbaoreau, A., 1S2, 40a
Chelae, 329.
Chemo-receptors, 314, 317, 319^ 325, 326.
C^A, 531.
Child, 254, 257, 332.
CMimpanxte^ motor cortex of, 272-279
Chloroform, So» Si, 1 13, 254, 28a
Ci^i^^ 32a

^^>

• 4»i^

Md iiic i iiii bic^ 14, 18L
17. A

Cott|«Ktml fcflez, 23S.
CoBfivmmatorj reactions, 329-333.
Cotttiacture, 248, 303, 304.

hcmiplmc. 248, 303, 304.
Contrast, 20S, ^7.

Coortfination, contribntioii to^ by re-
ceptor, 13.
Coonfinatioii in the simple reflex, 8» 36-
11&
in the compoond reflex, 114-234.
in the simuhaneoQS compoond re-
ilex (reflex >«ttBnt) i5o-i8a
in the reflex seqoence, 181-234.
Corda trmpani, 8c.
Corneal reflex, 2^

Cortex, cerebral, see ** Cerebral cortex."
CW^fc*, 85, 10^ 23S> 250^ 29s 3>9>
Croak-reflex, la

Crossed extension-reflex of 1^, 10^ 72,
77. 78. 79^ 83. 93* 99-»<». ««. "i,
»«7. i35-"37. 140. 143. «47. «5^
161, 102, 164, 16^ 189-191, 206-
209, 222, 224, 225.
Crossed stepping-reflex, 189^ 190^ 224.

INDEX

40s

Curare, iir

Cushing, H., 277.

Cushny, A^ iii.

Cybulski and Zanietowski, 70, 59S.

T. €700,99,113, 193, 396.

Darkness in visual field, 367-368.
Darwin, C, 10, 235, 250, 257, 260, 306,

396.
Darwin and reflex purpose, i, 10, 235,

236^ 306.
Decerebrate rigidity, 89, 251, 299-305,

De£aecation, 3061
Degeneration* Wallerian, 18.

method of snccessive, 52-54.
Deglutition, 69, 99^ 181, 182, 322, 326,

DeiterVnuoeas, 343.
D^jerine, 278.
Demoor, J., 399.
Dendrites, 39^ 41.
Depressor nerve, 99, 113.
Descartes, 256, 286. 287, 395.
Diaphra^, 67, 68, 205.
Diaschizis, 246.

Differentiation and integration corre-
lated, 344.
Diffuse nervous system, 31 1-312.
Disgust, 261-263.

Disunce-receptors, 306, 324-328» 330,
^333-335.390. ,. ^

the cerebrum the ganglion of, 349-
_. 353. 390.
Disynaptic arc, 54.

Donaldson. H.. 144, 145, 152. 334, 40a
Double-jomt muscles, 106, in, 283.
DouUe-sign reflexes, 83, 135, 199.
Duchenne, 161, 396.
Duval, 24.

Ea r thwor m , 182, 328.
EchinuSj 112, 314.
Eckhard, C, 9.
Edinger, L., 349, 402.
Edkins, J. S., 3, 402.
Effective threshold, 309.
Effector, 7, 309.
Ehriich, P., 141, 232, 236.
Elasmobranch, 349.
Emotional reaction, 255-268.
End-effect, 6.

rhythm oU 14* 42-66.

intensity of, 14, 70-79.
End-plate, 55.
Erb, 396.
.fij«r, 327.

5'^\J;. ^^' ^S. 246^ 338» 398.
Exaubility, selective, 13.

Exner, S., 15. 21, 87, 124, 113, 175. 181,

279. 28s. 396,^, 398. ^
** Extensor-thrust," 67-69, 74, 88, 90, 91,

93, 107, 146^ I47» 169. 174. 238,

248.
Exteroceptive field, 1 29-1 31, 316-322,

324, 341, 345.
Extero-reflexes, 129-133, 316-324.
Eyeball movements, 274, 275, 277, 279-

281, 285-286, 289, 337, 384, 385-
Eyelid-reflex, 45.

Fano, 398, 401.

Fatigue, 14, 214-223.

Fechner. 372, 375, fio, 395.

Fechner*s paradox, 375.

Ferricr, D., 270, 271, 272, 280, 295, 396^

^. ,. P^' 398.

Fick, A., 70.

Figure, reflex, 164-168.

Final common path, 55, 11 5-149, 223,

23^
relatively indefatigable, 223.

Flechsig, P., 277, 278, 279. 396, 401.

Flexion-reflex, 19, 21-14, 71, 73, 74, 78,
79» 83. 86, 8&-^, 103. 104, 107-
100, 127, 128, 131, 132, 134, 136,
138-140, I4> 147, I49-I52* "S*-
160, 102, 164-1^, 169, 173. '79^
185, 187-191, 198, 203, 204, 206,
208, 21WIQ, 224, 229, 240, 243,
244, 248. 288, 355, 3<6.

Flicker-sensation, binocular, 357-386 ;
asymmetrical, 368-371 ; symmet-
rical, 362-368 ; alternate, 163-^,
372, 379; synchronous, 103-167.

Flicker-sensation, imiocular, 363-368.

Flourens, 270, 395.

/7r, 238, 250.

Focus of effect of a reflex, 150, I5i» 239.

Foster, M^ 14, 309.

Franck. Fr., 19. 87, 397.

Franz, 307.

Fredericq, L., 239.

Freusberg, 396.

V. Frev, 12, 11, 226, 398.

Fritsch and Hitzig, 271, 273, 396.

/njf, 10, 128, 181, 238, 239. 249. 251,322,
327. 328. 330. 340, 342, 343. 349.
352.

Frohlich, A., 100, 253, 400, 401, 402.

Gad, J. 39.397.398. ^

and Joseph. 15,398.
Gall-bladder. 11.
Ganglia, antennary of Cardnus. 15.

spinal. 14, 320.

sympathetic. 15. 321.
Gaskcll, W. H.. 193. 194, 19S. 325. 397-
van Gehuchten, IS 85, Mi-

Hteni^ E^ :tf^ oj* :m zfld^ ^IJB^ 53
Httiiill. H. H^ :cr. i^a ail* ar^ ^li^

licmClL CTT^ :2H^ 3J3; TS5. 4D1.

and ArtKa^^ira .:ri* JPEI.

Eitiv E44, 14^ jai^

J™"^ _J^U* ^^K^

J*^^W J^l^f J^^

LDDBCBuQiSKr GDBuUCtSDB*. ^k^

IxHi^umd* VQ^ 1297; ogih.

IlZWOSlliC '',"^^*^^M.H1il^^ L4^ E3^ ^k'-^Em,

B«^ror. C^ aid. 27^^ 27^, apt; jgfc

Hubcr. C^ ,|00.

Fingnihig5>Jackgap. ^7x2^ 2d(V 303; 304*

3J4, J«^399-
fciiinter. J^ 143, ^ 5.
Hyde, Ida% 402.

Ooidtiiiigs^ 270, aSgb. 303, 304*

James. W, 39^ 258. ig^ afi^ jfi^r 3^-

^ law (^ gomord coDiinciaaB*'' 3^ 39.
ramixu ■I47'-

202^ 2Q5-29&.
JemiiasBk. r^S 39r^
** Jerli ** phrniMii i a ni^^ SS^ Sq^ 132^ su; 247,

ir^. 3P2^ ^
JoimstDix, 4C0.

INDEX

407

Joseph, Gad and, 15, 398.
Jurin, 372.

Kalischer, 157, 402.

Ketten-reflexe (Loeb) 182, 335.

Kicking, 306.

Kiesow, 226, 401.

Knee-jerk, 86-89, ^y, «", 247, 278, 302,

" Kno(£o'at " blow, 343.

Koster, and A. Tschennak, 99i 113, 400.

Krause, 276.

Kronedcer, H., 45, 99^ 182.

and S. J. MeUxer, 99, 182, 298, 397.

and W. Stirling, 45, 3^
Kiihne, W., 38.

Labyrinth, 133, 204, 246, 334, 335. 33^

344,348-
Lacertaf 322.
Ladd, G. T., 255, 40a
Lange, 258, 259, 265, 397
Langelaan, T. W., 402.
Langendorff, O., 92, 349, 396, 398.
Langley, 14, 3»8, 400.

and H. K. Anderson, 38, 398.
Lans, Zwaardemaker and, 45, 40a
Laryngeal nerve, saperior, 100.
Laslett, £. £^ 10, 50, 54, 401.
Latent period, 14, 18-26, 92.
Latzko and Sternberg, 254, 401.
Lauder Brunton, 193.
Law of Bell and Blagendie, 18, 791
Law of forward direction of conduction

(James), 38, 39.
Law of Talbot, 371. 372, 379, 38a
Laws of reflex-action, of Pfliiger, 76,

161-164.
Lee, F. S., 205, 398.
V Lenhossek, 85, 141.
Lewandowsky, 248, 304, 349. 401, 402.
Lewis, Mitchell and, 175, 397.
Lloyd Morgan, 237, 265, 268, 331, 390,
, ,.399.400.

Locahzation m motor cortex, 270-307.
"Local sign" in reflexes, 124-127, 248-

251.
Lockjaw, 295-299.
Locomotion, 64, 68, 69, 212, 213, 305,

334-336, 344, 350.
and receptive range, 334-336.
Loeb, J., 181, 205, 335. 399. 400, 401.
Lombard, W., 175, 397, 398.
Lotze, 235.
" Lower***^ and " higher " as applied to

organisms, 236-237.
Luciani, L., 272, 304, 349.
Ludwig, C, 1591
Lyon, 205, 400, 401.

Macallom, A. B., 198, 402.

Macdonald, J. S., 17, 68, 196-198, 199,

400, 402.
MacdousaOl, W., 200-203, 223, 367, 373,

382, 383, 386. 400, 401, 402.
MacwiOiam, J., 400.
Magendie, 38, 270, 395.
Magnus, R., 63, 400, 401.
MalapUrurus^ 38, 66^ 74.
Man, motor cortex of, 276.
Mann, 304, 398.
Mann, G., 272.
March of spinal and cortical reaction,

28a
Marey, E., 45, 396.
Mark-time reflex, 210-212.
Martin, Newell, 172, 397.
" Material me," 324.
May, Page, 40a
Median line, reflexes of, 322.
Median sagittal plane, relation of cortical

reactions to, 2S9, 384-385.
Medusa, 18, 3^42, 45, 50, 6i-%4, 69, 168,

169,249,250,312.
Meltzer, S, J., 99, 182, 397, 399.

and Kronecker, H., 298, m.
Membrane at cell-junctions, 10^ 17.
Membrane, synaptic, 42.
Memory, 228, 331, 332, 352.
Mendelsohn, M., 40a
Merzbacher, L., 71, 40a
Mesencephalo spinal path, 329, f 3CX
Metamerism, 314-3*6, 320^ 321, 344,

345-
Milieu interne, 4.
Mimetic movement, 354-367.
Minimum wsihile, 185.
Mislawski, 205, 40a
Mitchell and Lewis, 175, 397.
V. Monakow, 54, 136, 246, 273, 278,

401.
Monkey, hand of, 329.
Monti, 24.

Moore, B., and Reynolds, 15, 399-
Morgan. Lloyd, 237, 265, ^ 33«. 390»

399,400.
M068O, A., 182, 396.
Moth, 327.
Motor area of cerebral cortex, 271-306,

384,385,389.
Motor neurone, 55, 309.
Mott, F. W., 272, 279. 281, 289. 329, 385,
#9. 398. 402.

and Schafer, E. A., 281, 289.
MuUer, J., 377, 383. 395.
Munk, H.. 272, 397. 398.

and Obresia, 280, 398.
Miinzer and Wiener, 398, 401.
Muskens, 205, 401.
Mustelus, 329.

408

INDEX

Nagd. yr» tB, lag^ 105, 313, 398.
riAniMi F«y o5» 397*
NtrvMfl»39-4i>Mi3i4'
KmrogUa, 14.

WtHMTOBIH t C llllif Cdlt| IKMt

Ntwront* motofi 5$, 141, 14a.

thiwliold, 17, 1)5, 156.
Ntttronei, aoioeooia noveniMit of, 14.
NeutralitY of rdlexM, 146^ I47i ^ ^»90^

KtwtirMaftlii, 17a, 397.
Nfwton, In 377» 395-
Nkotlii, 14, 15.

NlMl,a47.

NocHDepthft rtltant, 91, 116-430, 1481

Nod-cepiM 13, laS-Mo^ 318, 330^ 345.
Kotimagd,6s,a54»39^

Obregb, Munk and, aSo^ 39B.

Oeiopbngeu reflfK, s8a.

Oeitrum, J63.

01d;ig, *}i, 3*^8.

f Mf at to phrenic afCj 151.

Optninu gf tncjutb* i^ ^^S"^^

Optic chiMiQiK, 377^ 383. .
Opiic nerve, 70, 1 45*334- ,
Ontm^tt/itni-^ mouir cortex oL 179-179^
(>tnryi.t, ic^; 1 69 .J J4t 335> 33^ 337*340^

Ott, I., 397.
OJWn. >4. 79-

Painoendingt, 326-339, 319.
Pain nerves, 226-229, 251* ^S^*

path in spinal cord, 251-254.

skin, 233-228.

visceral, 11.
Pallio spinal path, 329, 330.
Paneth, 272.
Panum, 382.

Parallelism of ocular axes, 352, 384, 385.
Paralysis, after cortical lesion, 377, 278.
Parasites, 63, 184, 238.
Pari, G.,ji.
Parker, G. H., 128,401.
Path, final common, 11 5-1 49.
Path, principle of the common, 11 5-149,
310.

private, 115, 116.
Pattern, reflex, 164-171.
Pawlow, J., 402.
Perceptual image, 347, 35^. 357-
Perikarya, 14, 15, 22, 82, 83.
Permeaoility of synaptic membrane, 42.
Perspective figures, 171.
Pfliiger, E., 7^ x6i, 235, 395,

76, i6i«-t6i|.

l^Pb 40Cy JOK.

PhocbHTOoepCon^ 333, 333^ 337, 50.

Fhreoie

Phrank reflex, 205.

£^,343- ^

Piioniotor nenree, 96i«
Pinnarreflex, 10^ 91.
PioCnmakj, 85. 105, 3^
Plnriodliiur oondoclof^ 39.
Plitriieoq;rtiTe wimmatkm, 1x3-1x7, 399^

31a
Pliiri8q[iiie&tal diMliBme, 139^
Pluria^iiMiital integiaSott, 314, 315, 344»

Polxiiwa condnctloiiy 3^

PdUmxiiti, O^ 398.

Porter, Townaead, 1449 399^

PoalKMnitral oofivuliitioB« 17A '7S ^77*

Poatnnu reflexaa, aa4, ^o^ 231, 337-

345'
Poitnres, aegmental and tolxly 3^* 34*~

Poatbig, 2U,

Pre-ceDUmfGoiiTc^iitioii, a7a-a8a
Precorrent leacdona, 336^ 3i9-33a.
Pr^otent reflexes, 324, aat-xy^ ^m,
Prendence of oootootii 3^» 37^ 3f^^*

Principle of the oobubob mO, ii5-«49^

<33» 310^ 34^ 35ir 3PS-
Prind^le of oompefitloii for eoeigy

(BfaodoqgaUl aoo-4(^ 367*
Ptoprio^qptiTe field, 139^ 130^ 304. 30S»

3'^ 3'7, 3^ 3««. 33^^S 347-

349.

Proprioceptive reflexes, 129-132, 204,

^ .305. 356,317, 3»-
PropnoK:eptive system, 336-345, 347-

. 349-

mcludes labyrinth, 336-345.
Proprioceptors, 130, 131, 320, 336-345-
Proprio-spinal nerve-tracts, 52-54.
Psendaffective reflexes, 251-254.
Psycho-physical parallelism, 386.
Pulmono-phrenic arc, 351.
Purpose in reflexes, 235-239, 305.
PuiTinp, 255.
Pyramidal tract, 329, 330.

Ramon v Cajal, 15, 24, 85, 141, 145, 232,

280,377,383,385. ^ ^

Reception, 6, 9-13, 309, 310, 316, 318,

Receptive ^Ids, 46, 90, 126-131, 157, 160,
174. 3«6-322.
exteroceptive, 130, i3»»3»7-3"9t320,
. 3" » 322,. 343.
mteroceptive, 317, 318.

INDEX

409

ReoepdTe fields, propruHxpdve, 129,

130, 204, 205, 316, 317, 320, 321,

,33^345. 347-349.
of extensor-thrust, 127.
of flexion-reflex, 90,01, 128, 131, 132.
of scratch-reflex, 4!^ 121, 126^ 128,

131, 132.

not identical with spinal root fields,

Receptive range, 333, 334.

Receptor, 7. 9. 12, 46. 61, 309, 310, 313,

3»8. 3»9. 323. 335, 336. 347.
a nurtor in co-ordination, 13.
Receptors, classification of, 31^319.
distance, 306^ 324-328, 330, 333-335.

390.
species of, 9-13, 130, 131, 225-229,

318-319.
symmetrica], 149, 122.
Reciprocal inhibition, 83-105.
Reciprocal innervation, 83, 84, 90-100,

and cerebral cortex, 279-299.

and muscular tonus, 3134, 305.
Reflex, the simple, 7, 8-1 13 ; an artificial
abstraction, 114, 115.

compound, 8, 114-234.
Reflex action, defined, c.

subiection to "volition," 300, 388,

Reflex-arc, 7, 46. 50-SS. ^S^, 30^3".

,3»..3V:
the primitive, 308-311.

Reflex attitude, see " Posture."

Reflexes, abdominal, 163.

adequate stimuli for, 9-13.

after-discharge of, 30, 33, 51, 74,
102-104. 288, 386.

allied, 119-13S ^67, 289, 310, 355.

alternating. 144, 200-203.

antagonistic, 135-149, 188-191, 205,
229. 28a 3>o. 311, 356.

as adapted reactions, 235-209, 300.

chain, 182.

compensatory, 144, 200, 203-205,

214. 337. 341.

croak, 10.

crossed extension, see ** Crossed ex-
tension reflex."

of double sign, 83, 135, 199.

extensor thrust, see "Extensor-
thrust."

eyelid, 45.

^gure, 164-168.

flexion, see *' Flexion-reflex."

focus of effect of, 150, 151, 239.

incremental, 22-25.

initial, 24.

intensity of, see " Intensity of reflex."

latency of, 14, 18-26, 92.

tail.

Reflexes, long, 157-166, 344.
mark-time, 210-212.
neutrality of, 146, 147, 289, 298, 306.
nod-ceptive, 226-230, 248, 252-254,

322,33?-332. ^^
pattern of, 164-168.
pinna, 10, 91.

postural, 204, 230, 231, 337-345-
refractory phase in, 14, 44-0^ 134.

»35-
rhythmic, 36. 45-^. "3^.
rhythm 6f discnarge m, 42-66^ 136,

»38-
scratch, see " Scratch reflex."
sequence, of, 180-234.
sexual, 210.
shake, 1^4, 2^.
short, 157-1^^321,344-
stepping, 65. 66^ 210-212.
swaUowmg, 69, 99, 181, 182, 322, 326,

29.333-
200, 210, 212, 223, 322.
tonic, 231, 301-305, 338-344. 348.
torticollis, 162.
type. 21, 65, 127.
union of, 347, 355, 356.
vasomotor, xi, 321.
visceral, 11. 317-320, 321, 322, 333.
whisker, 162, ^i.
Refractory phase m reflexes, 14, 44-69,

»34,i35-
Reinforcement, reflex, 175-180.
Reissner fibre, 329.
Renaut, 24.

Resistance, spinal, 109, 154-156.
Respiratory regulation, 205, 348, 390.
Restriction of distribution a {slcXot in

integration, 346-347.
Retrogradation, 267.
Reynolds and B. Moore, 15, 399.
Rhizostoma, 39, 61.
Rhythmic reflexes, 36, 45-66.
Rhythmic response firom cortex, 295.
Rhythmic response in reflexes, 42^DiS.
Richet, C, 36, 85. 105, w.
Rigor mortis, rapidity of onset of, 338.
Rohault, 377, 395.
Romanes, 18, 39, 41,45, 61, 118, 16',

Rontgen rays, 31a
Rosenthal. I., 193, 395,^99.

169, 249, 250,
Tays,

I. I..

Rotating lantern, 357-3^2.
Rothmann, 402.
van Rynberk, 402.

Salamonsen, J.. 400.

Sargent, P., 402.

Schafcr, E. A.. 43, 80, 272, 273, 280, 377,

397,398-
Mott, F. W., and, 281, 289.

4IO

INDEX

Sihftli^ulien (v. Monakow), 54, 136.
Schloesser, yjj.
Schopenhauer, 255.
Schreiber, 397.

Scratch-renex, 10, 20, JO, 36>4S-65»7ii
72, 75t 76. 78. 79^ 9»i toj» ng.

12CHI28, 131^ 135-140, 142, 145.

145, T47. 163. i73» »79. >«2, 183,
185-192* 199, 213. 214, 216-321 »
238 » 239, 244, 245, 248, 288.

Seemann, J , 400.

Segment, nervous mtegration of the^jig-

Segmental arrangement of nervous sys*

tcni, J 1 4-3 1 6,
of motor cortex, 277,
Segmental postures, 327, 342-343, 345.

346.
Segmental reflexes, 204, 230, 231, 337-

345-
Segments, neural integration of series

01,314-316,344,345*
leading, 323-324.
Selachian^ 330.

Selective excitability, 13, 227, 316, 318.
Self regulation of respiratory arcs, 99.
"emicircular canals, 336-338.
ensation, projicicncc of, 324, 325, 331,

^ , .343, 390..

Sensitivity of viscera, 11, 12,318.

Sensual fusion, 357-386.

Sensual "objects," 347, 357.

Sensual percept, 347, 357.

Separation of cells, 1 5^ 310.

Sergi, 259. 398. 399.

Setschenow, 36, 65, 395, 396,

Sexuat reflexes, 230, 326.

Shake>rel!ex, 164, 23]^

Shock, spinal, 14, 150, 240-24S, 352.

Siluroid fishes, 129,

Simia saiyrus^ motor cortex of, 273, 276-

279-
Simple reflex, 7^1 1 J.
Simultaneous combi nation of reflexes,

150-180.
Skeletal muscles, tonus of, 304, 338-339^
Smith -Kastncr, 395.
Snarling, 252, 255.
Sowton, S. C, M., 80, Si, 402,
Spallanzani, tTp, 395.
Species of reflex, enect on prcpotence,

226-231.
Spencer, Herbert, 257, ^44, 348, 397.
Spinal tnduiuon, immediate (or dire

^76, 119-132, 184, 185, 386.
successive (or indirect), 151, 206-
^ . ,-'3-387-
Spmal nerveroor, afferent, 85, 1 70, 174,
25'. 30^ 3^9* 3^0.
efferent, 319, 320.

direct J,

Spinal nerve-root, co-ordination and the,

170-174. 251,391.
Splanchnic. 100, 176.

Spodc,397. „ ^

"Spontaneous" reflex, 1531 aoSp 309.

Stannitis, 107.

Starling, E. H., Bayliss and, 3, 2j6, 312,

. . 399'
btefani, 304-
Steinach, K., 9, 14, 39^^
Stepping-reflex, 65-^ 210-2IZ,
Stcrnbere, M,, 132, 175* 398-

Latzko and, 254, 401.
Stewart, C, 40a
Stimulus, adequate, 12, 13, 91*

eflScacy 01 electrical, 13, 226.

intensity of, 14, 27, 30, 22^-22%, 231.

mass as a, 316,

nocuous, 13, 91, 226-230, 248» 252-
254p 322, 330-332-

*» object "as a, 347.357*

prolongation of, 30, 329.

threshold value of, 12, 208, 323-325.
Stirling, W., 36, 37, 93, 396.

with H. Kronecker, 45.
Storey, A., 401.
Strychnine, 71, 106-112,

160, 172, 292-299, w.
Sulcus antraiift genua of, 2;

«32» '54, «S9p

91. 93t 119. tJS.

356.

, 182, 322, 326^

Summation, 14, 36-3S,
310-311, 347, 35]
Swallowing, 69, 99, i\

329, 333-
Swinton, 2^8, 350. 397.
Sympathetic system. 318.
Synapse, 17, 18, 22, 24, 25, 42, 321.
setting of the, 24, 141, 143, 321.
different kinds of, 299.
different resistances at, 155, 1516.
an instrument of cc>-<.»rdination» 14O1
141, 310,311,321.328,351.
Synaptic conduction, 17, t8, 42. 140-142,

154-156.
Synaptic membrane, 16, 17,42, 141-J42.
Synaptic nervous system, 311-314.
Syncytia, 15,
Synergy, 178-1791

Tail, 200, 210, 212, 223, 322,

Talbot's law, 371, 372, 379, 38a

Tamburini, 272.

Tango-receptors, jig, 322, 335, 341.

Teleology and physiology, t, 235-269^

I
I

306.
Tetanus toxin, 109-113,

ndik<

t6c^ 292^299,

Thomdike, 307,
Threshold, effective, 309,

selective. 226, 227, 318, 319.

variability of, 14, 37,

INDEX

411

Threshold, of neurone, 155, 156.
Threshold-stimulus, 12, yog, 310.
Tlaropsis indUans, 1 18, 249, 250*
Tonic reflexes, 231, 301-305, 338-340,

► of attitude, 340- U4. 348.
Tonus-labyrinth, 133, 246, 336-344.
Tonus, reflex, of skeletal muscles, 86, 87,

88, 301-305. 338-340.
Topolanski, 284, 399.
TiMrpedo, 74.
Tortoise, 238, 325.
Touch-^pots, 230, 324.
Traube, 395.

Trauma, as a stimulus, 241-244.
Tremor, 214.
Trismus, 295-299.
TrogiodyUs gorilla, motor cortex of, 272-

279.288-
TVoglodyUs fjizer, motor cortex of, 272-

279, 288, 29a
Tschermak, A., 99. "3. "99. 236, 279,

329. 383. 399. 402.
and Koster, 99, 113, 40a
Tschiriew, 397.
Tkmcate, 62, 224.

V. UexkiUl, 112, 307, 314, 399, 40a
Unity of a motor centre, 76-78.
Uspensky, 142, 396.

Vagi, 260, 264, 287.

Valerius, 372, 374, 39^.

Vasomotor reflexes, 241-243, 258, 359,

265, 321.
Vertebrate, 62, 69, 84, 315, 320, 336.
Vcrwom, M., 2, 14, 79, 100, 141, 195, 309,

,r.v . 399. f», 401.
Vibnssae, 162, 252, 325, 341.

Viscera, sensittvity of, 11, 12.

reflexes from, 69, 314, 321.
Visceral field, 317-318, 352.
Vocalization, reflex, 252, 254, 255.
Vogt, 279.
Volkmann, 287, 395.
Vorticilla^ 6^ 309, 31a

Waller. A. D., 70. 80, 87, 398^ 3991

Wallerian degeneration, 18.

Walton, 71, 397.

Ward,;., 250,397.

Warren, Bowditch and, 175, 398.

Warrington, W. B., 318, 402.

Wasp4arvae, 331.

Weber, E. H., 287, 376, 380, 395.

Weber- Fechner rule, 376, 38a

Wernicke, 304.

Westphal, 87, 398.

Whiskers, 162. 252, 325, 341.

White rami of sympathetic system, 318.

Whytt, R., 24a

Wiener and Munzer, 398, 401.

"WUled" movements, 285, 286^ 306^

387-390.
Winckler, 40a
Winslow, 161.
Winterstein, 14.
Wollaston, 377. 395-

Woodworth, R. S., 251, 254, 295, 401, 402.
Wundt, 15, 3C^ 71, 193, 199, 304, 396^

397.

Yerkes, R. M., 307, 400, 401, 402.

2Umietowski and Cybulski, 70, 398.
Ziehen, 383, 401.

Zwaardemaker, 69, 123, 182, 400, 40I.
and Lans, 45, 399.

^ lANE MEDICAL UBRARY
•TjyMt*.^ 300 Pastbuk D»ivb
f^jff^^vto. CautoIkia 94304

Ignof9»c» ^ libraiys rotes does not exempt
' ■fiuiMtori from peinlties.

M>8 i^ ? Q/l

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